a symbol for the speed of light. One of the fundamental principles of physics is that light always travels at the same speed in a vacuum, exactly 299 792 458 meters per second or about 670 617 300 miles per hour. Another fundamental principle is that no object can travel faster than light. At speeds that are large fractions of the speed of light, the theory of relativity predicts a variety of strange physical effects. In calculations involving relativity, speeds are customarily expressed as fractions of the speed of light, such as 0.95 c.
C [1]
the Roman numeral 100, sometimes used as a unit of quantity or as a prefix meaning 100, as in Cwt (hundredweight) or CCF (100 cubic feet).
C [2]
a symbol for international standard paper sizes, followed by the size number, as in C4. The C series of sizes is used primarily for envelopes. Wikipedia has a description of this scale.
C [3]
a unit of relative current for batteries. For a particular battery, a current of 1C is a current in amperes numerically equal to the rated capacity of the battery in ampere hours. In other words, a 1C current will completely charge or discharge the battery in one hour.
abbreviations for "cranking amps" and "cold cranking amps," respectively. These units are often seen on motor vehicle batteries in the U.S. The amps involved are ordinary amperes of electric current. "Cranking amps" measure the current supplied by the battery when starting the vehicle at a temperature of 32 °F (0 °C), while "cold cranking amps" measure the current supplied at 0 °F (-17.8 °C).
a traditional unit of land area in Spanish speaking countries. In Spain and Peru the caballeria is equal to 60 fanegas, which is roughly 40 hectares (100 acres). In Central America it equals 60 manzanas, which is roughly 45 hectares (110 acres). In Cuba, the caballeria is a smaller unit equal to 33.162 acres or 13.420 hectares, but in the Dominican Republic it is a larger unit equal to 1200 tareas or about 75.4 hectares (186.5 acres). In Puerto Rico, the caballeria was equal to 200 cuerdas (see below) which is about 78.6 hectares (194.0 acres).
caballo de vapor (cv)
Spanish name for the metric horsepower.
a unit of distance formerly used at sea. The traditional U.S. mariner's cable was 120 fathoms long. This is equal to 720 feet, or 0.1185 nautical mile, or about 219.4 meters. The British Admiralty, in 1830, defined the cable to equal exactly 0.1 nautical (Admiralty) mile, which is 608 feet or about 185.3 meters. Some navies are now using a metric cable equal to exactly 200 meters (about 656.17 ft).
a traditional unit of volume in Jersey (Channel Islands), used for both liquid and dry commodities in trade. The cabot equals 10 pots [3], which is 17.375 Imperial quarts or about 19.747 liters. For dry commodities, the cabot is roughly comparable to 1/2 bushel.
an old name for a cask, sometimes used as a unit of measure for fish. A cade of herring, for example, was 720 fish.
calendar year (cal yr)
a civil unit of time, equal to 365 days or (in leap years) 366 days. See year [2]. In archaeology, climatology, and other sciences studying the earth over the last 40 000 years or so, a careful distinction must be made between calendar years (cal yr) and radiocarbon years (14C yr).
caliber (cal) [1]
a unit used to express the bore of a gun. (The bore is the inside diameter of the gun barrel.) Traditionally, the diameter was stated in inches, so that ".22 caliber" referred to a pistol having a bore of 0.22 inches (5.588 mm). This usage is declining, because bore diameters of many guns are now stated directly in millimeters. "Caliber" is the American spelling; elsewhere the unit is often spelled "calibre." The decimal point is usually omitted when caliber measurements are spoken (as in "38-caliber"). Sometimes it is omitted when the measurement is written, but this is not a good practice.
caliber (cal) [2]
a measure of the relative length of a gun barrel, defined as the length divided by the diameter of the bore. Thus a 50-caliber gun on a warship has a barrel 50 times longer than its bore. Confining the shell within the barrel for a longer time increases the velocity, so guns with a higher caliber usually have a longer range.
the thickness of a sheet of paper or card stock. Traditionally measured in points [7] (thousandths of an inch), caliper is now measured in microns (micrometers). The word "caliper" is sometimes used in place of the proper unit, as in ".004 caliper" (.004 inch or 4 points) or "120 caliper" (120 microns).
call second (Cs)
a unit of telecommunications traffic equal to one or more calls or other communications having an aggregate duration of one second. The call minute (Cmin) and call hour (Ch) are defined similarly.
Callipic cycle
a unit of time equal to 76 years or 4 Metonic cycles, formerly used in astronomy in predicting the phases of the Moon. After the passage of one Callipic cycle, the phases of the Moon repeat essentially on the same calendar dates as in the preceding cycle. The cycle is named for the Greek astronomer Callipus, who discovered it in 330 BCE.
calorie (cal)
the CGS unit of heat energy. This calorie (also called a gram calorie or small calorie) is the amount of heat required at a pressure of one atmosphere to raise the temperature of one gram of water by one degree Celsius. Unfortunately, this varies with the temperature of the water, so it is necessary to specify which degree Celsius is meant. A traditional choice was the degree from 14.5°C to 15.5°C; raising the temperature of water through this range requires 4.1858 joules, a quantity called the 15° calorie. Another choice produces the thermochemical calorie, equal to exactly 4.184 joules. More common today is the international steam table calorie, or IT calorie for short, defined by an international conference in 1956 to equal exactly 4.1868 joules, exactly 1.163 milliwatt hours, or about 0.003 968 32 British thermal units (Btu). The name of the unit comes from the Latin calor, heat.
Calorie (kcal or Cal)
a common name for the MKS unit of heat energy. This unit is properly called the kilocalorie; it is also called the kilogram calorie or large calorie. It is often (but certainly not always!) distinguished from the small calorie by capitalizing its name and symbol. The large calorie, or rather kilocalorie, is the amount of heat required at a pressure of one atmosphere to raise the temperature of one kilogram of water by one degree Celsius. Since this is 1000 times as much water as mentioned in the definition of the small calorie, the kilocalorie equals 1000 small calories, 4.1868 kilojoules, 3.9683 Btu, or 1.163 watt hours. (These conversions assume the IT calorie is in use; see previous entry.) These are the "calories" that joggers are trying to get rid of, the ones we gain by eating. The use of the same term "calorie" for two different-size units is endlessly confusing, but we seem to be stuck with it.
caña, canna, canne
traditional units of distance in Spain, Italy, and southern France, respectively. The caña varied in size, but it was most often defined as 8 palmos, which makes it the Mediterranean version of the fathom, equal to roughly 2 meters (6.5 feet). In Italy a measuring stick is still called a canna metrica. The unit is sometimes translated "rod" in English, but "fathom" is the proper choice.
candela (cd)
the SI base unit for measuring the intensity of light. Candela is the Latin word for "candle." The unit has a long and complicated history. Originally, it represented the intensity of an actual candle, assumed to be burning whale tallow at a specified rate in grains per hour. Later this definition was replaced with a definition in terms of the light produced by the filament of an incandescent light bulb. Still later a standard was adopted that defined the candela as the intensity of 1/600 000 square meter of a "black body" (a perfect radiator of energy) at the temperature of freezing platinum (2042 K) and a pressure of 1 atmosphere. This definition has also been discarded, and the candela is now defined to be the luminous intensity of a light source producing single-frequency light at a frequency of 540 terahertz (THz) with a power of 1/683 watt per steradian, or 18.3988 milliwatts over a complete sphere centered at the light source. The frequency of 540 THz corresponds to a wave length of approximately 555.17 nanometers (nm); normal human eyes are more sensitive to the green light of this wavelength than to any other. In order to produce 1 candela of single-frequency light of wavelength l, a lamp would have to radiate 1/(683V(l)) watts per steradian, where V(l) is the relative sensitivity of the eye at wavelength l. Values of V(l), defined by the International Commission on Illumination (CIE), are available online from the Color and Vision Research Laboratories of the University of California at San Diego and the University of Tübingen, Germany.
candle (cd)
an older name of the candela (see above), or of the candlepower (see below).
candlepower (cp)
a unit formerly used for measuring the light-radiating capacity of a lamp or other light source. One candlepower represents the radiating capacity of a light with the intensity of one "international candle," or about 0.981 candela as now defined. Since 1948 the candela has been the official SI unit of light intensity, and the term "candlepower" now means a measurement of light intensity in candelas, just as "voltage" means a measurement of electric potential in volts.
a traditional weight unit of South Asia. The candy was quite variable, generally within the range 500 to 800 pounds (225 to 365 kilograms). In the international cotton trade, the candy was generally equal to exactly 7 (British) hundredweight, which is 784 pounds or 355.62 kilograms.
can sizes
Food cans are identified by their nominal dimensions, diameter × height. (The "nominal" dimensions are somewhat larger than the actual dimensions, as is the case for lumber and some other products.) In the metric world the dimensions are 2- or 3-digit numbers representing dimensions in millimeters. In traditional U.S. nomenclature, the dimensions are stated as 3-digit numbers, with the first digit representing inches and the remaining two digits representing 16ths of an inch. A common can for fruits and vegetables, for example, is designated 83 × 116 in metric terminology, or 307 × 409 (3-7/16 × 4-9/16) in traditional terminology. If only one number is mentioned it is the diameter; thus a "404" can has a nominal diameter of 4-4/16 = 4.25 inches and a "65" can has a nominal diameter of 65 millimeters. Link: chart of standard can sizes with metric and traditional designations, from Dantraco Associates.
an English spelling for the Arab form of the quintal. In recent years, the cantar has been interpreted as a metric unit equal to 50 kilograms (110.23 pounds); traditional cantars tended to be a few percent larger than this.
in rowing, a "canvas" is the distance between the bowman and the bow, or between the coxswain and the stern. These areas were once covered by canvas. Winning by a canvas in rowing is analogous to winning by a head in a horse race.
Cape foot (cf)
a traditional unit of distance in South Africa. The Cape foot equals 12.396 English inches, 1.0330 English foot, or 31.4858 centimeters. This unit is not the traditional Dutch foot, but it is similar in length to the "Rhine foot" of northern Germany. The Cape foot was widely used for land measurement and appears on many deeds in South Africa. Europeans often referred to South Africa as "The Cape," meaning the Cape of Good Hope.
Cape rood
a traditional unit of distance in South Africa, equal to 12 Cape feet or 12.396 English feet (3.7783 meters). See also rood.
carat (ct or c) [1]
a unit of mass used for diamonds and other precious stones. Originally spelled karat, the word comes from the Greek keration, a carob bean; carob beans were used as standards of weight and length in ancient Greece in much the same way barleycorns were used in old England. Traditionally the carat was equal to 4 grains. The definition of the grain differed from one country to another, but typically it was about 50 milligrams and thus the carat was about 200 milligrams. In the U. S. and Britian, the diamond carat was formerly defined by law to be 3.2 troy grains, which is about 207 milligrams. Jewelers everywhere now use a metric carat defined in 1907 to be exactly 200 milligrams.
carat (ct or c) [2]
in Britain, the spelling "carat" is also used for the unit of gold purity known in America as the karat.
carb [1]
an informal unit used in the treatment of diabetes, equal to 15 grams of carbohydrates. This unit is known under various names, including carbo, carb unit, choice, or exchange. The significance of 15 grams is that in a very rough way that quantity of carbohydrate requires about 1 unit of injected insulin for patients with Type I diabetes (the actual ratio between carbohydrate and insulin varies considerably from patient to patient and is usually much lower for patients with Type II diabetes).
carb [2]
an informal unit equal to 1 gram of carbohydrate, commonly used in describing low-carbohydrate diets such as the Atkins diet. This newer usage of the term "carb" conflicts with the traditional use by diabetics (previous entry).
a former French unit for measuring the intensity of light. The unit was defined as the intensity of a standard Carcel lamp, which burnt colza oil in a precisely defined way. One carcel equals about 9.74 candelas (see above).
a traditional unit in Spanish and Portuguese speaking countries. The word means "load". It was often used as a unit of mass or weight equal to 3 quintals or as a unit of volume equal to the volume holding 3 quintals of the commodity being shipped.
carlength (or car length)
a unit of distance equal to the average length of an automobile. In the 1950s this was often estimated to be about 20 feet, but modern cars are shorter; midsize automobiles today average about 4.75 meters (roughly 15.6 feet). Drivers were formerly advised to allow one carlength for each 10 miles per hours of speed between their vehicle and the car ahead, but this has been replaced by the 2-second rule: allow a distance equal to 2 seconds of travel.
Carnegie unit
a unit of academic credit used in college admissions decisions in the U.S. The unit was introduced by the Carnegie Foundation for the Advancement of Teaching in 1914 to provide colleges with a standard measure of students' course work in high schools. A Carnegie unit represents the equivalent of one academic year of study in a subject in a class meeting 4 or 5 times a week for 40 to 60 minutes per meeting, a minimum of 120 hours of total class time.
a traditional unit of land area in Haiti equal to approximately 1.29 hectares (3.18 acres). The unit originated as the area of a square 100 pas (Haitian paces) on a side, with the pas being equal to 3.5 pieds (French feet). Foreigners are not allowed to own more than 1 carreau of urban land or 5 carreaux of rural land in Haiti.
a unit of volume, generally informal, equal to the capacity of a small cart. In Newfoundland, a cart of salt traditionally equalled 6 tubs or 108 Imperial gallons (490.98 liters).
a small container. The size of a carton is usually not standardized, but certain sizes are customary. In the U.S. citrus fruit industry, a standard carton is equal to 1/2 box or 0.8 bushel (28.191 liters).
another name for the hide, an old English unit of land area. The name comes from a Latin word meaning "plowland."
a conventional unit of sales for many items, varying with the item and over time. A case of wine, for example, is traditionally twelve 750-milliliter bottles. The word comes from the Latin capsa, a chest.
a unit of angle measure equal to 1/6 turn or 60°, used in mechanical engineering. A castellated nut is a locknut having a raised rim with a number of equally spaced slots, usually 6. A cotter pin fits into one of the slots and into a hole bored in the bolt, holding the nut in place with a precise degree of torque, or "tightness". To turn the nut one castellation is to turn it from one slot to the next, that is, by 1/6 turn.
category (cat)
the ranking of a hurricane on the Saffir-Simpson scale, used by the U.S. National Weather Service. A somewhat different scale of categories is used for tropical cyclones by the Australian Bureau of Meterology. Wikipedia has an excellent article on tropical cyclone scales comparing the descriptions used worldwide.
a weight unit of the colonial period in East and Southeast Asia, originating as the kati in Malaya. The catty varied a little from market to market. Typically it was equal to about 4/3 pound avoirdupois (604.79 grams), and it is still equal to that weight in Malaysia. In Thailand, the catty is used now as a metric unit equal to exactly 600 grams (1.3228 pounds). In China, the catty was identified with the jin, a traditional Chinese unit.
cawny or cawney
a traditional unit of land area in southern India, equal to about 4/3 acre (0.54 hectare). The name is an English tranliteration of a Tamil word for the unit.
abbreviation for "cubic," seen in combinations such as cbm (cubic meter) or cbft (cubic foot). The proper symbol for cubic meters is m3, not cbm.
cc [1]
an alternate symbol for the cubic centimeter (cm3). This symbol is obsolete and should not be used; cm3 should be used in its place. The cubic centimeter is the same volume as the milliliter (mL or ml).
cc or cc [2]
a symbol for the centesimal second (see below).
an abbreviation for 100 cubic feet. Local water and sewer utilities often sell water in CCF units; for this purpose one CCF equals about 748.05 gallons (U.S.) or about 2831.7 liters. Utilities sometimes sell natural gas in CCF units; for this purpose the CCF is really a unit of energy roughly equivalent to the therm (1 CCF of natural gas provides about 1.034 therm).
an incorrect symbol for the cubic centimeter (cm3).
a unit of telecommunications traffic equal to 100 call-seconds. One CCs could represent a single call 100 seconds long, or 10 calls each 10 seconds long, etc. The symbol stands for "centum call second."
CE [1]
abbreviation for "common era." This abbreviation is a non-religious designation used in place of the traditional AD for years of the common or Christian era. Years of the common era are supposed to be counted from the birth of Jesus of Nazareth, founder of the Christian religion. However, the year-numbering system was not established until more than 500 years later. It is based on calculations of the priest and scholar Dionysius Exiguus placing Jesus's birth in the Roman year 753 AUC. Dionysius knew he had incomplete information, and there is evidence that he picked this particular date to simplify the calculation of the date of Easter. According to the Biblical account, Jesus was born several years before the death of Herod the Great, who died, we now know, in 750 AUC (4 BC). Thus the calculation of the common era is off by 6 or 7 years at least. In the conventional use of the common era system, there is no year 0 and the year prior to 1 CE is designated 1 BCE (or 1 BC). In astronomy, however, it simplifies calculations to define the year 0 CE = 1 BC and to apply negative numbers to earlier years. Thus Herod died in -3 CE, and, in general, -n CE is the year more commonly called n + 1 BC.
CE [2]
French abbreviation for colonne d'eau, water column, seen in pressure measurements. See centimeter of water (below) or millimeter of water.
see degree Celsius. The word "degree" is often omitted in informal statements of temperature, as in "we expect a high of about 23 Celsius today."
Celsius heat unit (Chu)
a unit of heat energy equal to the energy required to raise the temperature of one pound of water by 1°C at standard atmospheric pressure. 1 Chu is equal to exactly 1.8 Btu, approximately 453.59 IT calories (see above), or 1.8991 kilojoules. The unit is also called the centigrade heat unit.
cent [1]
an old English unit of quantity, usually equal to 100 but sometimes 120 (the great or long hundred) or some other figure of similar size. The Latin number 100, centum, is also used in English works for this quantity.
cent [2]
an informal name for 1/100 of almost any unit, for example, a centiliter (0.01 liter). Phrases such as "15 cents of an inch" were formerly common in English.
cent [3]
a unit used in music when it is necessary to specify the ratio in frequency between two tones with great precision. There are 100 cents in a semitone, or 1200 in an octave. If two notes differ by 1 cent, the ratio between their frequencies is 21/1200 or approximately 1.000 5778. One cent equals 1/1.2 = 0.83333 millioctave or about 0.2509 savart.
cent [4]
a unit used in nuclear engineering to describe the "reactivity" of a nuclear reactor, equal to 0.01 dollar. For a discussion of reactivity, see inhour.
cent [5]
a common unit of land area in southern India, equal to 0.01 acre, exactly 435.6 square feet, or about 40.47 square meters.
cental (cH or ctl)
an alternate name in Britain for the U.S. hundredweight, which is equal to exactly 100 pounds (the British hundredweight is 112 pounds). Introduced by British merchants around 1850, the name was apparently coined after the model of the quintal. The cental has sometimes been confused with the centner (see below).
centesimal minute (c), centesimal second (cc)
units of angle measure sometimes used in surveying. In the centesimal system, the right angle is divided into 100 grads or gons. Each gon is divided into 100 centesimal minutes (or centigons) and each centesimal minute into 100 centesimal seconds. Thus the centesimal minute equals 0.009° or 0.54 arcminute, and the centesimal second equals 0.324 arcseconds.
centi- (c-)
a metric prefix meaning one hundredth, or 0.01. The prefix comes from the Latin word centum for one hundred.
centiare (ca)
a metric unit of area. The centiare equals 0.01 are, which is exactly 1 square meter (about 10.7639 square feet).
centibar (cbar or cb)
a metric unit of pressure identical with the kilopascal (kPa). One centibar equals 0.01 bar, 7.5006 torr, or 0.1450 pounds per square inch (lbf/in2 or psi). The centibar is traditionally used in agriculture as a unit of soil water tension (the water pressure on the roots of plants) as measured by devices called tensiometers.
centigon (cgon)
a unit of angle measure equal to 0.01 gon, 0.01 grad, or 0.0001 right angle; this is equivalent to 0.009°, 0.54 arcminute, or exactly 32.4 arcseconds. The centigon is useful in navigation (potentially, at least) because 1 centigon of latitude represents approximately 1 kilometer on the earth's surface, in the same way that 1 nautical mile represents approximately 1 minute of latitude in traditional navigation.
centigrade [1]
a temperature scale; see degree centigrade.
centigrade [2]
a French unit of angle measurement equal to 0.01 grad, 0.009°, 0.54', or 32.4".
centigram (cg)
a metric unit of mass equal to 10 milligrams or about 0.154 grain.
centigray (cGy)
a unit of radiation dose equal to 0.01 gray or 1 rad. Dose of radiation used in cancer treatments, formerly stated in rads, are generally stated now in centigrays.
an informal unit of pressure equal to 1 centimeter of mercury. This is equivalent to 10 millimeters of mercury, approximately 0.3937 inHg, 0.1933 lb/in2, 13.33 millibars, or 1333 pascals. The word is pronounced "sentig".
centiliter (cl or cL)
a common metric unit of volume. One centiliter equals 10 cubic centimeters; this is about 0.610 24 cubic inch, 0.3318 U.S.fluid ounce or 0.3519 British fluid ounce. In the kitchen, a centiliter is roughly equal to 2 U.S. teaspoons (or 0.704 British tablespoonful).
centimeter (cm) [1]
the basic unit of distance in the CGS version of the metric system, equal to 0.01 meter. One centimeter is about 0.393 700 787 inch.
centimeter (cm) [2]
an obsolete name for the statfarad (approximately 1.11 picofarad), the CGS electrostatic unit of capacitance.
centimeter (cm) [3]
an obsolete name for the abhenry, the CGS electromagnetic unit of inductance. The abhenry is the same as the nanohenry.
centimeter (cm) [4]
one of several traditional units of pressure, including the centimeter of mercury and the centimeter of water (next two entries).
centimeter of mercury (cmHg, cm Hg)
a traditional unit of pressure equal to 10 mmHg, 1.333 22 kilopascal, or about 0.193 pounds per square inch.
centimeter of water (cmH2O, cm WC, cm CE, cm WS)
a unit of pressure equal to the pressure exerted at the Earth's surface by a water column (WC) 1 centimeter high. This is about 98.067 pascals, 0.980 67 millibars, 0.3937 inch of water, or 2.04 pounds per square foot. The unit is used in respiratory medicine and elsewhere to measure air pressures. The French symbol is cm CE (colonne d'eau), and the German symbol is cm WS (Wassersäule).
a word sometimes used to mean 100 million (108). This usage applies the traditional Latin prefix cent-, meaning 100, but it conflicts with the metric prefix centi-, which means 1/100. In metric language, the number 100 million could be called a hectomillion or (at least in the U.S.) a decibillion.
centimorgan (cM)
a unit of genetic separation used in genetics and biotechnology. If two locations on a chromosome have a 1% probability of being separated during recombination in a single generation, then the distance between those locations is one centimorgan. In humans, the centimorgan is approximately equal to one million base pairs. The unit honors the pioneering American geneticist Thomas Hunt Morgan (1866-1945), who received the 1933 Nobel Prize in Medicine for his discoveries concerning the role played by the chromosome in heredity.
centinewton (cN)
a metric unit of force equal to 0.01 newton. This unit has some popularity in engineering as a substitute for the gram of force (gf), since it equals about 1.019 72 gf (about 0.0360 ounces of force in the English system). In the textile industry, the breaking strength of fibers is commonly expressed in centinewtons per tex.
centipoise (cP, cPs, or cPo)
a common metric unit of dynamic viscosity equal to 0.01 poise or 1 millipascal second (mPa·s). The dynamic viscosity of water at 20 °C (68 °F) is about 1 centipoise. The correct symbol for the unit is cP, but cPs, cPo, and even cps are sometimes used. Both "centipoise" and "centipoises" are in use for the plural.
a unit of angle measure equal to 0.01 radian or about 0.572958° (34' 22.65").
centisecond (cs or csec)
a unit of time equal to 0.01 second or 10 milliseconds. Centiseconds are frequently used in the study of human speech to measure precisely the length of sounds.
centistokes (cSt)
a common metric unit of kinematic viscosity equal to 0.01 stokes or 1 mm2/s. The viscosity of lubricating oils and many other liquids is frequently stated in centistokes. Although the centistokes is not an SI unit it is likely to remain in use, since it provides a convenient and traditional name for an SI-appropriate quantity (1 mm2/s). The older symbol cS should not be used for this unit, since S is now the symbol for the siemens.
centner [1]
the English name for a German weight or mass unit, the zentner, equal to 50 kilograms or about 110.231 pounds. The name centner should not be used for the cental (see above).
centner [2]
a Russian weight or mass unit equal to 100 kilograms (approximately 220.4623 pounds). This centner, also used in Ukraine and the other former Soviet republics, is equal to the decitonne and to the metric quintal; it is twice the size of the centner [1] used in western Europe.
an informal name for the centiradian (see above).
the Latin number 100, sometimes used in works in English.
century [1]
a unit of quantity equal to 100. In ancient Rome, a "century" was originally a company of about 100 soldiers led by an officer called a centurion.
century [2]
a traditional unit of time equal to 100 years. In naming centuries, historians recall that there was no year 0 in the conventional year numbering system. Thus the First Century included the years 1-100 and the Twentieth Century included the years 1901-2000. (As an example in the other direction, the Fifth Century BC included the years 500-401 BC.) With this convention, 2001 is the first year of the Twenty-first Century.
cetane number
a measure of the ability of diesel fuel to reduce engine knocking. The cetane number plays the same role in diesel engine technology that the octane number plays in conventional automobile engine technology. It is the percentage by volume of cetane which must be added to methylnaphthalene to give the mixture the same resistance to knocking as the diesel fuel sample being tested. Cetane is the name of a hydrocarbon compound whose molecules contain 16 carbon atoms and 34 hydrogen atoms, the 16 carbons being arranged in a long chain. Adding one oxygen atom to cetane produces cetyl alcohol, a waxy compound found in whale oil. The words "cetyl" and "cetane" are both derived from the Latin word cetus for a whale.
a common symbol for the cubic foot.
a symbol for cubic foot of gas equivalent, used in the natural gas industry. This is really an energy unit; 1 cfe is equal to about 1034 British thermal units (Btu), 0.01034 therm, or 1.091 megajoule (MJ). Multiples of this unit are formed using non-metric prefixes: Mcfe for 1000 (not 1 million) cfe, MMcfe for 1 million cfe, and Bcfe for 1 billion cfe.
cfh, cfm, cfs
traditional abbreviations for cubic feet per hour, cubic feet per minute, and cubic feet per second, respectively. 1 cfm = 28.3169 liters per minute (L/min) and 1 cfs = 28.3169 liters per second (L/s). Symbols with a slash, such as Cf/h, are also seen.
abbreviation for "colony forming units," a count of the number of active bacterial cells in preparations of Lactobacillus acidophilus and other "friendly" organisms of the digestive system. Counts as high as one billion CFU per gram are not uncommon.
chain (ch)
a unit of distance used or formerly used by surveyors. Although the unit is not often used today, measured distance along a road or railroad is commonly called chainage regardless of the units used. The traditional British surveyor's chain, also called Gunter's chain because it was introduced by the English mathematician Edmund Gunter (1581-1626) in 1620, is 4 rods [1] long: that's equal to exactly 1/80 mile, 1/10 furlong, 22 yards, or 66 feet (20.1168 meters). The traditional length of a cricket pitch is 1 chain. Gunter's chain has the useful property that an acre is exactly 10 square chains. The chain was divided into 100 links. American surveyors sometimes used a longer chain of 100 feet, known as the engineer's chain or Ramsden's chain. (However, Gunter's chain is also used in the U.S.; in fact, it is an important unit in the Public Lands Survey System.) In Texas, the vara chain of 2 varas (55.556 ft) was used in surveying Spanish land grants. In the metric world, surveyors often use a chain of 20 meters (65.617 ft). See also shackle and shot [2] for anchor chain lengths.
chain number
a size designation for roller chains, such as the drive chains of bicycles or motorcycles. These chains are traditionally designated by a three-digit number. The first digit specifies the pitch, the distance between pins, in eighths of an inch; the second and third digits specify the width of the rollers in 80ths of an inch. Thus a 425 chain has a pitch of 4/8 (= 0.5) inch and a roller width of 25/80 (= 0.3125) inch.
chalder or chaldron (chd)
a traditional British unit of volume or weight used for dry commodities such as coal or lime. As a volume measure, the chaldron is equal to 36 bushels, or 288 British Imperial gallons; this is equivalent to 46.237 cubic feet or 1.3091 cubic meters. As a measure for coal, the chalder equals 1/8 keel or 53 hundredweight (5936 pounds or 2692.52 kilograms). The words "chalder," "chaldron," and "cauldron" are English spellings of the same old French word, which originally meant a large kettle.
champagne quart
see quart [2].
character (char)
a unit of information used in computer science and telecommunications. The number of bits required to specify a character has varied as more robust coding systems have been developed. By the 1970s, one character was typically equal to 8 bits or one byte. Current coding systems such as UTF-8 use up to 4 bytes to designate all the characters used in all the world's languages and other symbolic systems.
a traditional Russian unit of volume containing about 123.0 milliliters, 4.159 U.S. fluid ounces or 4.329 Imperial fluid ounces. There are 6.25 charki in a boutylka (bottle) and 10 in a schtoff. The word charka means a cup or glass.
Charrière, Charrière gauge (Ch)
a unit of distance used for measuring the diameters of small tubes such as catheters, fiber optic bundles, etc. The gauge number is the diameter of the tube in units of 1/3 millimeter. In English-speaking countries, the scale is usually called the French gauge and the unit is simply called "French." Charrière was a nineteenth century French instrument manufacturer.
cheval vapeur (cv or ch)
French name for the metric horsepower. In Canadian French, however, the term is used to mean the English horsepower.
chǐ, chi or ch'ih
a traditional unit of distance in China, sometimes called the Chinese foot. In modern China, the chǐ equals 10 cùn, exactly 33-1/3 centimeters, or 13.123 356 inches. There are 1500 chǐ in a .
chiliad [1]
a unit of quantity equal to 1000. The word comes from the Greek numeral 1000, chilioi, which is also the origin of the metric prefix kilo-. Pronounced "killiad," the chiliad was once fairly common in learned writing, but it has nearly disppeared from use today.
chiliad [2]
another name for a millennium (1000 years).
an informal Italian name for a kilogram.
Italian spelling of the metric prefix kilo- (1000).
one of several spellings in English for the jin, a traditional Chinese weight unit.
in New Zealand, another name for a punnet.
a name used in Thailand and Laos for the viss. The word is sometimes spelled joi in English.
a traditional French unit of volume. The unit varied regionally, but by the 18th century it was more or less standardized as 23.475 cubic pouces (465.7 milliliters). The chopine is obsolete in France today, but the word survives (especially in Canada) as a French name for the English pint units.
a traditional Scottish unit of volume equal to 2 mutchkins or 1/2 Scots pint. The choppin is equivalent to about 52.1 cubic inches, 1.80 U.S. liquid pints, 1.50 British Imperial pints, or 854 milliliters.
CI or ci
a traditional symbol for the cubic inch.
cicero (cc)
a unit of distance used by typesetters and printers in continental Europe, equal to 12 Didot points. This is approximately 0.1780 inch or 4.52 millimeters. The cicero corresponds to the British and American pica. Presumably, this unit got its name because type of this size was used in printing the works of classical authors such as the Roman statesman and orator Marcus Tullius Cicero (106-43 BCE).
a symbol for cubic inch of displacement, formerly used in the U.S. in stating the engine displacements of motor vehicles; these measurements are now made in liters.
an old English word for the number 5, pronounced "sink" and derived from the French number 5, cinq. In English history, the original Cinque Ports were Sandwich, Dover, Hythe, Romney and Hastings. The word survives today as the name for a 5-spot showing in dice, or for a 5-card in card games.
circle (cir)
the traditional unit of angle measure, divided into 2π radians or 360 degrees.
circular inch, circular mil (cmil)
informal units of area. A circular inch is the area of a circle one inch [1] in diameter, and a circular mil is the area of a circle one mil [1] in diameter. A circle of diameter d has an area of π·d2/4, so the circular inch is equal to approximately 0.785 398 square inches or 5.067 07 square centimeters, and the circular mil is equal to approximately 0.785 398 square mils or 506.707 square micrometers.
city block
see block.
civil year
a year as measured by the conventional (Gregorian) calendar, equal to 365 days in most years but 366 days in a leap year. See year [2]. This is the same unit as the calendar year (see above). Both names are often used to specify years beginning with January 1, as opposed to a fiscal year beginning on some other date.
Clark degree
see degree [4].
a unit of entropy. Entropy is a measure of the extent to which heat or energy in a physical system is not available for performing work. It is computed in units of energy per kelvin. One clausius is equal to 1 kilocalorie per kelvin (kcal/K) or 4.1868 kilojoules per kelvin (kJ/K). The unit honors the German physicist Rudolf Clausius (1822-1888), who introduced and named the concept of entropy in 1850.
U.S. military slang for the kilometer (about 0.621 mile). Also spelled klick or klik. This unit became popular during the Vietnam War, but it was invented by U.S. troops in Germany during the 1950s. Occasionally it was used as a non-metric unit equal to 1000 yards (0.9144 kilometer).
city block
see block.
a unit of thermal insulation used for clothing. One clo is intended to represent the insulation required to keep a resting person warm in an indoor room at 70 °F (21.1°C). The rate of a person's heat loss is measured in watts per square meter of skin area per kelvin of temperature difference across the clothing; the value of insulation is measured by the reciprocal of this rate, in square meter kelvins per watt (m2K/W). One clo is equal to 0.155 m2K/W or 1.550 togs.
clothyard, clothier's yard
an alternate name for the ell. The English ell is 45 inches (1.143 meters), but the "clothyard arrows" used with longbows in late medieval times were closer in length to the 37-inch Scottish ell.
an old English unit of weight. A clove is usually considered equal to 1/2 stone or 1/16 hundredweight; that's 7 pounds (3.175 kilograms) by the modern definition of the stone, but in the past the clove varied from 6.25 to 8 pounds.
a unit of power used to express the performance or leakage of vacuum pumps. One clusec represents a flow of 10 milliliters per second at a pressure of one micrometer (or micron) of mercury. This is equivalent to 0.01 lusec or 1.333 microwatts. The name of the unit is an acronym for "centi-lusec."

Rudolf Clausius (1822-1888)
German physicist
a unit of relative electric current used especially in connection with nickel metal hydride (NiMH) storage batteries. The symbol designates the current flow per hour, into or out of the battery, as a fraction of the battery's rated capacity. In other words, a current of 0.1 CmA would completely charge or discharge the battery in 10 hours. Put another way, if the rated capacity of the battery is 2 ampere hours or 2000 milliampere hours, then 0.1 CmA is a current flow of 200 milliamperes.
symbol for the circular mil (see above). Note that this is not a centimil.
coffee measure
a flat-bottomed scoop or spoon used to measure coffee in U.S. homes. The coffee measure holds 2 U.S. tablespoons (about 29.57 milliliters).
a unit of volume formerly used in U.S. food recipes. A coffeespoon is 1/2 teaspoon, 1/12 fluid ounce, or about 2.5 milliliters.
an ancient Persian unit of liquid volume, equal to 1/8 artaba or (in recent centuries) about 8.25 liters.
color rendering index (CRI)
a scale used in engineering to measure the ability of an artificial lighting system to show the "true" colors of objects, that is, the colors those object display in natural daylight out of doors. The scale is from 0 to 100, with higher numbers representing a higher degree of fidelity of color. The test procedure, developed by the Illuminating Engineering Society of North America (IESNA), involves comparison of eight test colors under both natural lighting and the artificial source being tested.
color temperature (CCT)
a measure of the overall "color" of a light source. The measurement is obtained by comparing the spectrum, or mix of colors (wavelengths of light), produced by the light source to the spectrum of a "black body," a theoretical object that absorbs all radiation falling on it. (A black body is also a perfectly efficient radiator of energy, its spectrum depending only on its temperature.). The measurement is expressed in kelvins (K). Lower temperatures indicate more red and yellow light, higher temperatures more blue. Incandescent light bulbs are "cool" with a color temperature of about 2800 K, while daylight at noon is much "hotter" (bluer) at about 6000 K. Fluorescent lighting can be produced with a broad range of color temperatures including everything between these two values. "CCT" stands for "correlated color temperature."
color units
several systems have been devised to measure colors. For most of us not directly concerned with dyes, paints, or inks, the subject was academic until recently, but now computers require precise methods for describing the colors to be displayed or printed. These methods typically use three variables, reflecting the fact that the human eye has three types of color sensor. Computer monitors use the RGB system, which specifies colors with three variables measuring the intensity of the three primary colors red, blue, and green in the color. Frequently each variable is specified by one byte and therefore takes values in the range 0 to 255. If all three are 0, the resulting color is black; if all three are 255 the resulting color is white. The RGB settings for the Carolina blue background of this page are R=153, G=204, and B=255. Since it is difficult to estimate the relative amounts of red, green, and blue needed to create a particular color, many graphics design programs use the HSV color system, which describes colors using three variables called hue, saturation, and value. Once again, all three variables are assigned values from 0 to 255. Hue, which is what we call "color" in ordinary language, is described on a circular scale. Hue values begin with red at 0 and run through yellow, green, blue, and purple before returning to red at 255. Saturation is the purity of the color, the extent to which it is not watered down with gray. The pure color has saturation 255. As saturation is reduced, the color becomes grayer, until at saturation 0 the color is replaced by a neutral gray of the same intensity as the original color. The value (or intensity) of the color is its brightness. The pure or most natural form of the color has value in the middle of the scale, at 127. As value is increased the color becomes brighter. In the opposite direction, the color becomes less bright, becoming black at value 0. This page's background has hue 140, saturation 240, and value 192 in the HSV system. See also: Lovibond color units (used for beer and honey).
colpa, colp, or collop
a traditional Irish unit. The colpa was originally a unit of livestock equal to one cow or horse or to 6 sheep. Later it was used as a unit of pastureland equal to the pasturage supporting one colpa of livestock. This varied according to the quality of the land, but it was roughly equal to the Irish acre (0.6555 hectare). "Collop" is an English version of the Irish word "colpa."
column inch (col in)
a unit of relative area used in journalism. A column inch is an area one column wide and one inch deep. The width of a column varies; a standard size in the U.S. is 2-1/16 inch. At this width a column inch is 2.0625 square inches or about 13.31 square centimeters.
commercial acre
a unit of area used in U.S. real estate, equal to exactly 36 000 square feet or about 0.826 45 ordinary acre (0.334 45 hectare). The unit was invented by commercial realtors to express the approximate portion of an acre of subdivided land that remains to be sold in the retail market after portions are set aside (dedicated) for necessary streets and other utilities. Buyer beware! It is legal to sell land by the commercial acre in many U.S. states, although most consumers are not aware of the smaller size of the unit.
common year
a year of 365 days, as opposed to a leap year of 366 days.
cooling degree day (CDD)
see degree day.
cone, cone number
a measure of temperature used by potters. Pyrometric cones are cone-shaped objects designed to soften and bend after absorbing a specific amount of heat. Potters place these cones in the kiln and observe them through peepholes; when the cone bends all the way over the proper amount of heat has been delivered to the pottery being fired. Although cones bend within narrow temperature ranges there is not a simple relationship between cone number and temperature. Technical tables are posted on the Internet by the Orton Ceramic Foundation.
a metric unit of area used in Vietnam. One cong equals 1000 square meters, which is 0.1 hectare, 0.24177 acres, or 1196.00 square yards.
a historic unit of liquid volume. The Roman congius was equal to about 3.2 liters (3.4 U.S. quarts or 2.8 British Imperial quarts); it was divided into 6 sextarii (sixths) which corresponded closely to modern pints. In the nineteenth century, the congius was used in British medicine and pharmacology as a name for the British Imperial gallon (4.546 09 liters).
coomb or coom
a traditional British unit of volume used mostly for dry commodities. A coomb is 4 Imperial bushels; this is equivalent to 5.1374 cubic feet or about 145.48 liters.
an abbreviation for coefficient of performance, a measure of the efficiency of heat pumps, air conditioners, refrigerators, and freezers. The COP is the ratio of the useful energy output of the system (the amount of heat energy added to or removed from the building) divided by the electric energy input when the unit is operating in a steady-state test condition. Typical values are in the range 2-4. (The energy "output" exceeds the input, because the system only uses the input energy to move heat energy, not to create it.) Heat pumps have a higher COP for heating than for cooling, because the compressor's input energy produces heat energy that can be used in heating but must be dissipated outdoors for cooling. For air conditioners, the COP equals the energy efficiency ratio (EER) divided by 3.412.
cord (cd) [1]
a traditional unit of volume used to measure stacked firewood. Like most traditional units of trade, the cord has varied somewhat according to local custom. In the United States, the cord is defined legally as the volume of a stack of firewood 4 feet wide, 8 feet long, and 4 feet high. (In Maryland, the law specifies that the wood be stacked "tight enough that a chipmunk cannot run through it." Presumably it is up to the buyer to provide the chipmunk.) One cord is a volume of 128 cubic feet, about 3.6246 cubic meters, or 3.6246 steres. The name apparently comes from an old method of measuring a stack of firewood using a cord or string.
cord (cd) [2]
in the U.S. timber industry, the cord is also used as a unit of weight for pulpwood. The weight varies with tree species, ranging from about 5200 pounds (2.36 metric tons) for pine to about 5800 pounds (2.63 metric tons) for hardwood.
cord foot (cd ft)
a traditional unit of volume used to measure stacked firewood. A cord foot is the volume of a stack of firewood 4 feet wide, 1 foot long, and 4 feet high. Thus the cord foot is 1/8 cord, or 16 cubic feet, or about 0.4531 cubic meter.
cordel [1]
a traditional unit of distance in Spain and Latin America. More specifically, the cordel is a rope used in land measurement. In Mexico and the southwestern U.S., the cordel measured 50 varas or about 42.33 meters (138.9 feet), using the Texas standard of 33 1/3 inches for the vara. In Cuba, however, the cordel was only 24 varas or about 20.35 meters (66.8 feet). Longer cordels were used in some parts of South America.
cordel [2]
a unit of area equal to one square cordel: about 1792 square meters (2143 square yards or 0.433 acre) in Mexico and the southwestern U.S. or about 414.2 square meters (495.4 square yards).
coulomb (C)
the SI unit of electric charge. One coulomb is the amount of charge accumulated in one second by a current of one ampere. Electricity is actually a flow of charged particles, such as electrons, protons, or ions. The charge on one of these particles is a whole-number multiple of the charge e on a single electron, and one coulomb represents a charge of approximately 6.241 506 x 1018 e. The coulomb is named for a French physicist, Charles-Augustin de Coulomb (1736-1806), who was the first to measure accurately the forces exerted between electric charges.
count (ct) [1]
a unit of quantity equal to 1. This unit is used in commerce to specify that the quantity stated represents a reliable count. For example, a carton marked "oranges 24 ct" contains exactly 24 oranges.
count (ct) [2]
a traditional unit measuring the texture of a fabric, equal to the number of threads per inch. A 100 count fabric has 39.37 threads per centimeter.
count (ct) [3]
an informal unit of volume in bartending, equal to 0.5 fluid ounce (14.8 milliliters). Bartenders usually fit bottles with pourers designed to restrict the flow to 0.5 fluid ounce per second. They can then measure a quantity of liquid by counting, "one thousand one, one thousand two, ..." while pouring. This is much faster than using a measuring glass, and just about as accurate.
count (ct) [4]
a measure of size used in the U.S. for shrimp and similar items described by the number of items per pound. Thus "50 count" shrimp weigh an average of 1/50 pound each.

Charles-Augustin de Coulomb (1736-1806)
French physicist
English, like most languages, has a procedure for stating the precise relationship between persons of common descent; a typical designation is "second cousins, once removed." First cousins are persons sharing a common grandparent; second cousins are persons sharing a common great-grandparent, and, generally, for n > 1, n-th cousins are persons sharing a common (n - 1)-times-great-grandparent. This means n-th cousins have n + 1 generations in each of their descents from the common ancestor. The "removed" phrase is used when the number of generations in the descent from the common ancestor is not the same for both cousins: "r times removed" means the difference in the number of generations is r. Thus, for n-th cousins r times removed, the common ancestor is an (n - 1)-times-great-grandparent of one cousin and an (n + r - 1)-times-great-grandparent of the other cousin. (In the case of first cousins r times removed, the grandparent of one cousin is also an r-times-great-grandparent of the other cousin.) For n-th cousins r times removed, there are n + 1 generations in the descent from a common ancestor for one cousin, and n + r + 1 generations for the other. The number of degrees of consanguinity between n-th cousins r times removed is 2n + r + 2.
covado, covido
Portuguese and Arabic names, respectively, for the cubit (see below). The Portuguese covado is equal to 3 palmos (66 centimeters, or 20.12 inches), while the Arabic covido is about 48 centimeters or 19 inches.
a traditional Welsh unit of area, standardized in the British system to be exactly 2/3 acre (about 0.2698 hectare). The word is an Anglicized version of the Welsh name cyfair for the unit.
a common abbreviation for characters per inch, used in printing. The unit is also called pitch.
a traditional abbreviation for cycles per second; also an incorrect symbol for the centipoise (see above).
a traditional unit of volume formerly used by fishermen. The cran, originally intended to represent the volume of fish held by a barrel, was standardized at 37.5 Imperial gallons, or about 170.46 liters. This is somewhat larger than the usual Imperial barrel. In modern times, the cran is also used somewhat informally to mean a volume of fish weighing about a hundredweight (112 pounds or 50.8 kilograms). The name of the unit is an old Gaelic word meaning a quantity or measure of something.
an old English unit of volume for grains. Never standardized, the crannock was roughly 10 bushels (350 liters).
a unit of mass sometimes used in the physics and chemistry of gases. The crith is equal to the mass of a liter of hydrogen at standard temperature (0.01°C) and pressure (1 atmosphere); this is about 89.885 milligrams. The name comes from an ancient Greek word for a barleycorn.
a traditional unit of quantity in India, equal to 107 or 10 million. Large numbers are usually described in India using the crore and the lakh (105); for example, the number 25 600 000 is called 2 crore 56 lakh and written "2,56,00,000".
a unit of relative time in music equal to 1/4 whole note or 1/8 breve. The word, pronounced crotch-it, comes from the old Norse word krok for a hook; in this context it refers to the traditional hooked symbol for a quarter note.
a unit of information in computer science, equal to 2 bits. The unit is thought to have originated at IBM in the early 1980s. There are 2 crumbs in a nibble.
abbreviation for "cubic," as in cubic feet or cubic meters. This symbol is forbidden by the SI, but it remains in common use in ordinary English text. The proper symbol for cubic feet is ft3, not cu. ft.
cuadra [1]
a traditional Latin American unit of distance. The cuadra is generally equal to 100 varas (about 84 meters or 275 feet) in Central America and northern South America. In Argentina and Chile, the cuadra is equal to 150 varas (roughly 130 meters or 410 feet).
cuadra [2]
a traditional Latin American unit of area equal to one square cuadra [1]. Except in Argentina and Chile, this is 10 000 square varas, generally in the range 1.75-1.85 acres (0.71-0.75 hectare). In Argentina and Chile, the cuadra was 22 500 square varas (4.18 acres or 1.69 hectare).
a traditional Spanish unit of volume comparable to the liter or the English quart. The cuartillo equals 4 octavillos or 1/4 almude and contains 1.156 25 liters, which is about 1.222 U.S. liquid quart or 1.017 British Imperial quart.
cubic centimeter (cm3 or cc)
the CGS unit of volume, equal to 10-6 cubic meter, 1 milliliter, or about 0.061 023 7 cubic inch.
cubic foot (ft3, cu ft, or cf)
a traditional unit of volume in English speaking countries. One cubic foot equals 1728 in3, 1/27 yd3, 0.028 316 85 m3, or 28.316 85 liters. The cubic foot also holds about 7.4805 U.S. liquid gallons or about 6.2288 British Imperial gallons.
cubic hectometer (hm3)
a unit of volume equal to one million cubic meters. This unit is used commonly in reservoir and water supply management. One cubic hectometer equals 100 hectare meters, about 810.71 acre feet, or 35.3147 million cubic feet.
cubic inch (in3, cu in, or CI)
a traditional unit of volume in English speaking countries. One cubic inch equals 1/1728 = 5.787 037 x 10-4 ft3, 16.3871 cm3, 16.3871 milliliters, 0.5541 U.S. fluid ounce, or 0.5767 British Imperial fluid ounce.
cubic kilometer (km3)
a unit of volume equal to one billion (109) cubic meters. This unit is used in geology and geophysics to describe volumes of materials such as ice or lava.
cubic meter (m3)
the SI unit of volume, equal to 106 cm3, 1000 liters, 35.3147 ft3, or 1.307 95 yd3. A cubic meter holds about 264.17 U.S. liquid gallons or 219.99 British Imperial gallons.
cubic mile (mi3, cu mi)
a unit of volume used occasionally for very large volumes such as the amount of rock blasted out by major volcanic eruptions. One cubic mile is equal to about 4.168 181 87 cubic kilometers or exactly 147 197 952 000 cubic feet.
cubic ton
a unit of volume, used in various ways but most commonly equal to 40 cubic feet (1.13267 cubic meters). This makes it the same as the freight ton (see ton [5]).
cubic yard (yd3)
a traditional unit of volume in English speaking countries. One cubic yard equals 27 ft3, 46 656 in3, 0.764 555 m3, or 764.555 liters. A cubic yard holds about 201.97 U.S. liquid gallons or about 168.20 British Imperial gallons.
a historic unit of distance frequently mentioned in the Bible. The word comes from the Latin cubitum, "elbow," because the unit represents the length of a man's forearm from his elbow to the tip of his outstretched middle finger. This distance tends to be about 18 inches or roughly 45 centimeters. In ancient times, the cubit was usually defined to equal 24 digits or 6 palms. The Egyptian royal or "long" cubit, however, was equal to 28 digits or 7 palms. In the English system, the digit is conventionally identified as 3/4 inch; this makes the ordinary cubit exactly 18 inches (45.72 centimeters). The Roman cubit was shorter, about 44.4 centimeters (17.5 inches). The ordinary Egyptian cubit was just under 45 centimeters, and most authorities estimate the royal cubit at about 52.35 centimeters (20.61 inches).
cuerda (cda) [1]
a traditional unit of land area in Puerto Rico. The cuerda is equal to about 3930 square meters, 4700 square yards, 0.393 hectare, or 0.971 acre. Because the cuerda and the acre are so close to being equal, they are often treated informally as being equal. Mainlanders sometimes call the unit the "Spanish acre."
cuerda (cda) [2]
a traditional unit of distance in Guatemala, equal to 25 varas or about 21 meters (roughly 69 feet). Since cuerda means a cord or rope in Spanish, this unit probably arose as the length of a measuring rope. The cuerda is also used as an area measure equal to 1 square cuerda or 625 square varas; this is about 440 square meters or 527 square yards.
cuerda (cda) [3]
a traditional unit of volume for firewood in Cuba, analogous to the U.S. cord (see above). A cuerda of firewood is equal to 128 cubic pies, 2.87 cubic meters, or 0.79 cord.
an informal unit of flow equal to 1 cubic meter per second or about 35.3147 cubic feet per second. The name was coined after the model of the cusec (see below).
cùn, cun, or t'sun
a traditional unit of length in China, sometimes called the Chinese inch. In modern China, the cùn is equal to exactly 3-1/3 cm, or about 1.312 336 inches. There are 10 cùn in the chǐ or Chinese foot. The cùn is sometimes identified as the distance between the two outer folds in the bent middle finger, and this personal unit (called the "body inch" in English) is still used by traditional acupuncturists.
a measure of wood volume used in forestry. One cunit (pronounced cue-nit) is a volume of timber containing 100 cubic feet (2.8317 cubic meters) of actual wood (excluding bark and air between the logs). The unit is used mostly for wood intended as pulpwood or firewood.
cup (c) [1]
a traditional unit of volume used in recipes in the United States. One cup equals 1/2 (liquid) pint, or 8 fluid ounces. Technically, one cup equals exactly 14.4375 cubic inches or approximately 236.6 milliliters, not that anyone measures quite so precisely in the kitchen. American cooks use the same size cup for measuring both liquid and dry substances. In Canada, a cup is equal to 8 Imperial fluid ounces (13.8710 cubic inches or 227.3 milliliters). In Britain, cooks sometimes used a similar but larger unit called the breakfast cup, equal to 10 Imperial fluid ounces. The British cup equals 1/2 Imperial pint, but the Canadian cup is only 0.4 Imperial pint.
cup (c) [2]
an informal metric unit of volume equal to 250 milliliters, commonly used in recipes in Australia.
cup (c) [3]
an informal unit of volume for coffee. The size of a cup of coffee varies according to local custom, but a typical size is about 5 fluid ounces or 150 milliliters.
curie (Ci)
a unit of radioactivity. One curie was originally defined as the radioactivity of one gram of pure radium. In 1953 scientists agreed that the curie would represent exactly 3.7 x 1010 atomic disintegrations per second, or 37 gigabecquerels (GBq), this being the best estimate of the activity of a gram of radium. See also becquerel. The unit is named for Pierre and Marie Curie, the discoverers of radium.
a traditional unit of flow equal to 1 cubic foot per second or about 28.317 liters per second.
cut [1]
a traditional unit of length for yarn in Scotland and northern England. One cut equals 1/12 hank, a unit varying with the material. A cut of cotton yarn is 70 yards; a cut of wool is 46 2/3 yards.
cut [2]
a unit of concentration for shellac; see pound cut.
CV or cv
a common symbol for the metric horsepower, standing for the French name cheval vapeur or the Spanish name caballo de vapor.
Cwt or cwt
traditional symbol for the hundredweight.
a traditional symbol for the cubic yard.
cycle (c)
an informal name for "cycle per second." The frequencies of radio signals and of alternating electric current were previously stated in cycles; thus the alternating current in American homes is often described as "60-cycle" and a radio station might describe its signal frequency as "1040 kilocycles", really meaning 1040 kilocycles per second.
cycles per second (cps)
a traditional unit of frequency equal to one per second, or one hertz. Almost all measurements of frequency are now stated in hertz, the SI unit.

Marie Skłodowska Curie (1867-1934)
Polish-French physicist

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July 13, 2001