an international symbol for year, taken from the Latin word annus. Although English-speaking countries continue to use the traditional symbol yr for most purposes, scientists use the a symbol in papers and textbooks. The symbol is often seen in combinations such as Ma (million years) or Ga (billion years).
a or A
shortened version of am or AM in a statement of time (see below).
a symbol for international standard paper sizes, followed by the size number, as in A4 for a standard business-letter sheet. Wikipedia has a description of this scale. A sheet of size An has a width of 100·2-1/4-n/2 centimeters and a length of 100·21/4-n/2 centimeters (rounded to the nearest millimeter). Note: "A" is also the SI symbol for the ampere (see below).
see "angstrom star" below.
see "as" below.
a prefix indicating that an electrical unit is part of the CGS absolute electromagnetic system. These units are also indicated by the notation emu (as in "volt emu"). Although these units are defined naturally as part of the CGS system, all of them except the abampere and abcoulomb are either much too large or much too small for most applications. They have been replaced almost completely by the corresponding SI units. Following are the SI equivalents for each of the "ab" units:
Electric current: 1 abampere = 10 amperes
Electric charge: 1 abcoulomb = 10 coulombs
Capacitance: 1 abfarad = 109 farads = 1 gigafarad
Inductance: 1 abhenry = 10-9 henry = 1 nanohenry
Resistance: 1 abohm = 10-9 ohm = 1 nanoohm
Conductance: 1 abmho = 109 siemens
Magnetic flux density: 1 abtesla = 10-4 tesla = 1 gauss
Potential: 1 abvolt = 10-8 volt = 10 nanovolts
Power: 1 abwatt = 10-7 watt = 0.1 microwatt
Magnetic flux: 1 abweber = 10-8 weber = 1 maxwell.
absorbance unit (AU)
a logarithmic unit used to measure optical density, the absorbance of light transmitted through a partially absorbing substance. If T is the percentage of light transmitted, then the absorbance is defined to be -log10 T absorbance units. An increase in absorbance of 1.0 AU corresponds to a reduction in transmittance by a factor of 10. If the absorbance is 1.0 AU then 10% of the light is transmitted; at 2.0 AU only 1% of the light is transmitted, and so on. Note that AU is also used in astronomy as a symbol for the astonomical unit (see below).
abv, abw
symbols for alcohol by volume and alcohol by weight. 1% abv = 1% v/v and 1% abw = 1% w/v.
academic year
a unit of time in U.S. schools, generally equal to 9 months beginning in August or September.
accm, acfm, acfh, acfd, acim (etc.)
symbols for "actual cubic centimeters per minute", "actual cubic feet per minute", "actual cubic feet per hour", "actual cubic feet per day", and "actual cubic inches per minute." Many similar abbreviations are in use. These are units of flow rate for gases. See sccm.
a symbol for the "anatomical Chinese inch" or t'sun [2].
acoustic ohm
any one of several units measuring sound resistance. These units got their name by analogy with electric resistance, which is measured in ohms. The sound resistance across a surface in a given medium is defined to be the pressure of the sound wave at the surface divided by the volume velocity. Unfortunately, the result is often stated in "acoustic ohms" no matter what units are used to measure pressure and time. In the CGS system, the acoustic ohm equals 1 microbar second per cubic centimeter (µbar·s/cm3), which is the same as 1 dyn·s/cm5. In the MKS system, the acoustic ohm is equal to the SI unit, 1 pascal second per cubic meter (Pa·s/m3), or 1 N·s/m5. The CGS acoustic ohm equals 105 MKS acoustic ohms. See also rayl.
acre (ac or A)
a unit of area used for measuring real estate in English-speaking countries. "Acre," an Old English word meaning a field, is derived from the Latin ager and Greek agros, also meaning a field. The acre was originally defined as the area that could be plowed in a day by a yoke of oxen. It was in use in England at least as early as the eighth century, and by the end of the ninth century it was generally understood to be the area of a field one furlong (40 rods or 10 chains) long by 4 rods [1] (or 1 chain) wide. Thus an acre is 10 square chains, 160 square rods, 43 560 square feet or 4840 square yards. There are exactly 640 acres in a square mile. In metric countries the unit corresponding to the acre is the hectare, which is 10 000 square meters (the area of a square 100 meters on each side). One acre is equal to 0.404 687 3 hectare. Among traditional European land area units, the acre is typical in being defined as a day's work but unusual in not being visualized as the area of a square. Similar units include the French journal, north German and Dutch morgen, south German and Swiss juchart, Austrian joch, and Czech jitro.
acre (used as a unit of length)
various historical references mention "acre" as a unit of length, but there has never been any official definition of such a unit. In older works, especially in Britain, an acre of length is a furlong and an acre of breadth is 4 rods, since those were the historic dimensions of an acre. The acre is also the area of a square about 208.710 feet (roughly 208 feet 8.5 inches or 63.615 meters) on a side, and sometimes, mostly in the U.S. and Canada, this length was called an acre or the side of an acre. In contrast, the original area unit was sometimes called the square acre. All these usages are obsolete.
acre foot (ac ft or af)
a unit of volume used to measure the capacity of reservoirs. One acre foot is a volume one foot deep covering an area of one acre. Thus an acre foot contains exactly 43 560 cubic feet, about 325 851.4 U.S. gallons, or about 1233.482 cubic meters (0.123 348 hectare meter). The symbol af is widely used in reservoir management in the U.S., often in combinations such as kaf (1000 acre feet) or maf (million acre feet; this symbol should be Maf).
acre inch (ac in)
a unit of volume equal (of course) to 1/12 acre foot. An acre inch contains exactly 3630 cubic feet, about 27 154.29 U.S. gallons, or about 102.7902 cubic meters.
abbreviation for the Latin anno domini, "year of the Lord," the traditional designation for years of the common or Christian era. This abbreviation is often replaced by CE (common era), especially in countries where Christianity is not a dominant religion. See CE for additional information.
a suffix added to a number to create a unit of quantity equal to that number: for example, a 24ad is a unit of quantity equal to 24. Units of quantity equal to 1 through 8 are known, respectively, as the monad, dyad, triad, tetrad, pentad, hexad, heptad, and octad, terms coined by adding -ad to the Greek numbers 1-8.
Admiralty mile
see nautical mile.
a unit of time equal to one billion years (1 Ga). Proposed in 1957 for use in geology, the aeon is not approved by the SI and hasn't found much favor.
an abbreviation for annual fuel utilization efficiency, a measure of the efficiency of a gas furnace. The rating is designed to represent the percentage of the fuel energy actually delivered as heat energy, averaged over the course of a typical heating season. The actual calculation is quite complex, taking many properties of the furnace into account. Older furnaces have ratings of 60% AFUE or even lower; the newest high-efficiency furnaces are rated in the 90%-95% AFUE range. The U.S. Department of Energy requires new furnaces to operate at 78% AFUE or better.
agate, agate line
a traditional unit of area used in printing and advertising. The agate line is equal to 1/14 inch (1.814 millimeters) multiplied by the width of the printed line. Its name comes from the traditional type size called agate, which sets approximately 14 lines to the inch (very small print!).
abbreviation for the Latin anno hegirae, traditionally used in the West to designate years of the Islamic calendar. The Islamic calendar is lunar, counting twelve lunar months to the year (see month [2]). Thus its years are shorter than ordinary solar years. Years are counted from the day of the Hijra (Hegira in Latin), the flight of Mohammed from Mecca to Medina, which is fixed at 16 July 622 AD in the Julian calendar. The Islamic year AH 1440 begins at sunset of 10 October 2018 in the current (Gregorian) calendar. Link: Islamic calendar information from IslamicFinder.org.
air mass
a unit used in astronomy in measuring the absorption of light from the stars by the atmosphere. One air mass is the amount of absorption of light from a star directly overhead (at the zenith). The absorption of light from other stars is greater, because their light must pass obliquely through the atmosphere. If Z is the zenithal angle, the angle between the star and the zenith, then the absorption of its light is estimated to be sec Z air masses, "sec" being the secant trigonometric function.
air watt
an engineering unit used to express the effective cleaning power of a vacuum cleaner or central vacuum system. The air watt is practically the same as the ordinary watt. Measurements of vacuum power, however, are computed from English units using the following formula established by the American Society for Testing and Materials (ASTM): power in air watts equals 0.117354·F·S, or very nearly F·S/8.5, where F is the air flow in the system in cubic feet per minute (CFM) and S is the suction pressure in inches of water column (in WC). This definition makes the air watt equal to 0.9983 watt.
ale gallon
a traditional unit of liquid volume in Britain and the U.S., the ale gallon was equal to 282 cubic inches (4.6212 liters) or about 1.2208 U.S.liquid gallon (1.0165 British Imperial gallon). Standardized in the sixteenth century under Queen Elizabeth I, the ale gallon remained in use well into the nineteenth century but is obsolete today. It is also called the beer and ale gallon.
allergy unit (AU)
a measure of the potency of the compounds used by physicians in skin testing for allergies. The units are calibrated for each compound and the strength of the solutions is stated in allergy units per milliliter (AU/ml).
a traditional unit of volume in Spain and Portugal. The two countries used the name for units of quite different sizes. The Spanish almude is comparable to the British gallon; it holds about 4.625 liters, equivalent to 1.017 Imperial gallon or 1.222 U.S. liquid gallon. The Portuguese almude is much larger; it holds about 16.7 liters, which is 3.67 Imperial gallons or 4.41 U.S. liquid gallons.
aln or alen
a traditional Scandinavian unit of distance very similar to the north German elle: roughly 2 feet or 60 centimeters. The Danish alen, also used in Norway, was equal to 62.77 centimeters (24.71 inches). The Swedish aln was 2 fot or 59.38 centimeters (23.38 inches).
alpha TE
an abbreviation for "alpha tocopherol equivalent," a measure of vitamin E used in nutrition. Vitamin E is actually a group of related chemical compounds called tocopherols. The activity, or potency, of vitamin E in a food or food supplement is measured by the quantity (in milligrams) of alpha tocopherol (the most active of the forms of the vitamin) which would be equivalent to the compounds present in the food or supplement. One milligram alpha TE is equal to 1.5 international units (IU).
alqueire [1]
a traditional unit of volume for dry goods in Portugal and Brazil, equal to 1/4 fanga (the Portuguese equivalent of the Spanish fanega). This is about 13.8 liters or 12.5 U.S. dry quarts.
alqueire [2]
a traditional unit of land area in Portugal and Brazil. Still used commonly to measure farmland in Brazil, the unit varies considerably from region to region. One alqueire equals 2.42 hectares (5.980 acres) in São Paulo; 4.84 hectares (11.960 acres) in Río de Janeiro, Minas Gerais, and Goiás; 9.68 hectares (23.920 acres) in Bahía, and 2.7225 hectares (6.728 acres) in the northern part of the country. In Portugal, where the unit is obsolete, it was much smaller, about 0.14 hectare (0.35 acre).
alt h
traditional abbreviation in pharmacy for alternis horis, every other hour, a unit of frequency sometimes used in medical prescriptions.
am or AM [1]
abbreviation for the Latin ante meridiem, "before noon," used after a time to indicate that the time is before 12:00 (noon). The notations "am" and "pm" are used extensively in the United States, Canada, and other countries where time is usually not stated on a 24-hour basis. By convention, midnight is represented as 12:00 am and noon as 12:00 pm. In text, however, it is best to avoid the use of 12:00 am or 12:00 pm since the reader may not be aware of these conventions.
AM [2]
abbreviation for the Latin anno mundi, "year of the world." This abbreviation is traditionally used to designate years in the Jewish calendar, which counts years from the creation of the world as described in the Hebrew scriptures. The Jewish calendar is lunisolar; its years correspond to ordinary solar years (see month [2]). The first day of AM 1 is equivalent to 6 October 3761 BCE in the Julian calendar, and AM 5779 begins at sunset of 9 September 2018 in the current (Gregorian) calendar. Link: Wikipedia has a good description of the Jewish calendar and its history; and a date converter is available.
units used by physicists to express the relative volume and density of gases. The amagat volume unit is about 22.414 liters per mole (L/mol) or 0.022 414 m3/mol, the volume occupied by a gas at standard temperature (0.01°C) and standard pressure (1 atmosphere). The amagat density unit represents the corresponding relative density, which is equal to one kilomole per standard volume, or 44.615 moles per cubic meter (mol/m3), or 0.044 615 mole per liter (mol/L), again provided the measurement is made at standard temperature and pressure. In general, the ideal gas law shows that the relative density is equal to P/RT, where P is the pressure on the gas, T is the absolute temperature (in kelvins) and R is the universal gas constant, R = 8.314 joules per mole per kelvin. The unit honors the French physicist E. H. Amagat (1841-1915), whose work included the study of gases under pressure.

Émile Hilaire Amagat (1841-1915)
French physicist
an old English unit of volume, used for both liquids and dry goods. The amber was equal to about 4 bushels or roughly 140 liters.
American run
see run.
ampere (A or amp)
the SI base unit of electric current, named for the French physicist André-Marie Ampère (1775-1836), one of the pioneers in studying electricity. Until recently the official definition of the ampere went like this: suppose we have two parallel conductors, infinitely long and having negligible cross section. Place these conductors one meter apart in a perfect vacuum. One ampere is the current which, if it's flowing in these conductors, creates between them a force of 0.2 micronewtons per meter of length. (You're welcome to object that no one can make an infinitely long conductor, nor a perfect vacuum.) In November 2018 CGPM adopted a new definition by taking the fixed numerical value of the elementary charge e to be 1.602 176 634×10−19 coulomb. This is equivalent to defining the ampere as a flow of 1/e electrons per second; 1/e = 6.241 509 074×1018 approximately. The other electrical units are all defined in terms of the ampere. For example, one ampere of current results from a potential distribution of one volt per ohm of resistance, or from a power production rate of one watt per volt of potential. The unit is known informally as the amp, but A is its official symbol.
ampere hour (A·h or amp hr )
a commercial unit of electric charge often used to state the capacity of a battery. One ampere hour is the charge accumulated by a steady flow of one ampere for one hour. This is equivalent to exactly 3600 coulombs.
ampere per meter (A/m) or ampere-turn per meter
the SI unit of magnetic field strength. One ampere per meter is equal to pi/250 oersteds (12.566 371 millioersteds) in CGS units. The ampere per meter is also the SI unit of "magnetization" in the sense of magnetic dipole moment per unit volume; in this context 1 A/m = 0.001 emu per cubic centimeter.
ampere turn (At)
the MKS unit of "magnetomotive force." Electric current passing through a coil of wire generates a magnetic field. This field-generating ability is called magnetomotive force; it is equal to the product of the current, in amperes, and the number of turns of wire in the coil. One ampere turn equals 4pi/10 = 1.256 637 gilberts (Gb).

André-Marie Ampère (1775-1836)
French physicist and mathematician
a historic unit of volume. An amphora is the volume of an urn or jar of the same name. These urns were tall, with handles near the top on both sides (the word amphora comes from two Greek words meaning "on both sides" and "carry"). Amphoras were the containers of choice for shipping wine and many other commodities in the ancient world. Archaeologists report that the Greek amphora held about 38.8 liters (10.25 U.S. liquid gallons, or 8.54 British Imperial gallons). The Roman amphora was smaller, about 25.5 liters (6.74 U.S. gallons or 5.61 British Imperial gallons).
anatomical Chinese inch (ACI)
a name in English for the t'sun [2], a unit of relative distance used in acupuncture.
another name for the anker (see below).
angstrom (Å or A)
a metric unit of length, equal to 0.1 nanometer or 10-10 meter. Angstroms are used most often to measure the wave length of light waves. There is a technique called spectroscopy for identifying chemical substances by the wave lengths of light which they absorb (or emit, depending on the circumstances). Light shining through or from the substance is passed through a prism, which separates the various wave lengths to form a colorful spectrum. The spectrum often includes bright bands corresponding to wave lengths at which the substance emits light, or dark bands if the substance is absorbing light. The pattern of bands identifies the substance in somewhat the same way a fingerprint identifies a person. One of the pioneers of spectroscopy was the Swedish physicist Anders Jonas Ångström (1814-1874), and in his honor the wave lengths of light waves are customarily stated in angstroms. Although English speakers usually pronounce the word as if it were English, the Swedish pronunciation is closer to "ong-strerm."
angstrom star (Å* or A*)
a unit used to measure the wavelength of X-rays. Because it's easier to measure the ratio between two X-ray wavelengths than it is to measure the wavelengths themselves, the wavelengths are usually stated as multiples of a standard wavelength. The X unit and the angstrom star are the units used for this purpose. Å* was defined by J.A. Bearden in 1965 to provide a unit approximately equal to the angstrom (10-10 meter or 0.1 nanometer). Later measurements have shown that in fact A* is equal to approximately 1.000 0015 × 10-10 meter or 100.000 15 picometers.
angular mil
see mil [2].
animal unit (AU)
a unit of feed consumption used in U.S. dairying and ranching. One animal unit is the feed or grazing requirement of a mature cow weighing 1000 pounds (about 454 kilograms). This is approximately 26 pounds (11.8 kg) of dry forage. Total feed requirements are often figured by the animal unit month (AUM), the feed required to sustain one animal unit of livestock for one month (780 pounds or 354 kg).

Anders Jonas Ångström (1814-1874)
Swedish physicist and astronomer
anker or anchor
a small wine barrel used in Britain and elsewhere in northern Europe. In England an anker usually held 10 wine (U.S. liquid) gallons (37.85 liters); the Scottish anker held 20 Scots pints (about 34 liters). The word anker is of Dutch origin.
a unit of land area in South Asia. In Pakistan, the anna equals 20.17 square yards or 16.86 square meters. In Nepal, the anna is nearly twice that size: 31.8 square meters, 342 square feet, or 38 square yards.
annual percentage rate (% APR)
a unit used in the U.S. for stating interest rates and rates of return on investment. By federal regulation, these rates can be stated however a financial institution wishes, but they must be stated also in % APR so that consumers can compare rates of different loans and investment opportunities. Mathematically, the natural rate r of return on money is the "instantaneous" rate, the rate that allows for compounding of interest continuously. The APR is the percentage growth rate a of the money over a period of one year, as if interest were compounded annually. The two rates are related by the formulas a = er - 1 and r = ln (1 + a), where er is the natural exponential function and ln is the natural logarithmic function.
an abbreviation for "apothecary," used with certain units to indicate that the measurement is in the traditional apothecary system (see below). For units existing in both the apothecary and troy systems, the apothecaries' units are the same as the troy units, for example, 1 oz ap = 1 troy ounce.
Apgar score
a numerical measure of the health of a newborn baby. One minute after birth (and at regular intervals thereafter through the first moments of life) newborns are rated 0, 1, or 2 on five indicators of health (respiratory effort, heart rate, skin color, muscle tone, and reflexive response to smell). Possible scores therefore range from 0 to 10. The unit is named for its inventor, the American anesthetist Virginia Apgar (1909-1974).
API degree, API gravity
see degree API.
apostilb (asb)
an MKS unit of luminance, equal to the brightness produced by 1/pi candela per square meter or 10-4 lambert. The name of the unit is pseudo-Greek for luminance; it combines the ancient Greek stilbein, to "glitter" or "shine," with the prefix apo-, "away from."
apothecary weights
a version of the troy weight system formerly used by apothecaries (pharmacists). The troy pound, equal to 373.242 grams or 13.165 72 avoirdupois ounces, is always divided into 12 ounces, each ounce being equal to 480 grains. In the apothecary system, the ounce is divided into 8 drams (60 grains) each containing 3 scruples (20 grains). There is a parallel system of liquid volume measure in which the fluid ounce is divided into 8 fluid drams (or fluidrams) each containing 3 fluid scruples. The apothecary system continued in use into the early twentieth century, but it has been replaced everywhere in pharmacy by the use of metric units. The abbreviation "ap." indicates apothecaries' units, as in "oz. ap." or "dr. ap."
arcminute (′ or min)
a unit of angular measure, also called the minute of arc, equal to 60 arcseconds and to 1/60 degree. There are 21 600 arcminutes in a circle.
arcsecond (″ or as or sec or s)
a unit of angular measure, also called the second of arc, equal to 1/60 arcminute. One arcsecond is a very small angle indeed: there are 1 296 000 seconds in a circle. The SI defines s as the symbol for the time unit and recommends ″ as the symbol for the arcsecond. The symbol as has become common in astronomy, where very small angles are stated in milliarcseconds (mas).

Virginia Apgar (1909-1974)
American obstetrical anesthesiologist
are (a)
a unit of area equal to 100 square meters. The word is pronounced the same as "air." Being the area of a square 10 meters on each side, the are is a little large for measuring areas indoors and a little small for measuring areas outdoors. As a result, the are is not used as often as its multiple, the hectare (ha). One are is approximately 1076.3910 square feet, 119.5990 square yards, or 0.02471 acre.
in English the term "arm's-length" is used figuratively to mean "a distance discouraging familiarity or conflict." In many cultures, however, the length of a human arm is standardized as a unit of distance equal to about 70 centimeters or 28 inches. Examples include the Italian braccio, the Russian sadzhen, and the Turkish pik.
arpent [1]
a traditional unit of distance in French-speaking countries. The arpent equals 30 toises or 10 perches; this is about 191.8 feet or 58.47 meters. The unit was used to measure land; in fact, arpentage is the French word for surveying. In Canada the arpent has an official definition of 191.835 English feet (58.471 308 meters).
arpent [2]
a traditional unit of area in French North America (Québec and Louisiana), equal to one square arpent [1]. The arpent of area equals 900 square toises, 100 (square) perches, approximately 0.8445 acre or 0.3419 hectare. By the official Canadian definition, the arpent of area contains 36 800.667 23 English square feet or about 0.844 827 acre (0.341 889 hectare).
arratel or artel
versions of a traditional Arab unit of weight; see rotl. The spelling "arratel" is used in Portugal as an alternate name for the libra.
arroba (@) (1)
a traditional unit of weight in Spain and Portugal, equal to 1/4 quintal. However, the Spanish and Portuguese quintals are of different sizes. In Spain, the arroba equals 25.36 pounds (11.50 kilograms); arrobas of very similar sizes were established in the Spanish speaking countries of Latin America. In Portugal and Brazil, the arroba traditionally equals 32.38 pounds (14.69 kilograms, but in recent years this has been "metrized" to be exactly 15 kilograms). The arroba has also been used as a metric unit equal to exactly 15 kilograms. The name of the unit comes from ar rub', Arabic for "the quarter." The @ sign has been used in Spanish as a symbol for the arroba since the sixteenth century at least.
arroba (@) (2)
a tradiational unit of liquid volume in Spain and Latin America. This unit was intended to represent the volume of liquid weighing one arroba, so it varied with the liquid being measured; for example, an arroba of oil was 12.563 liters (2.972 U.S. gallons), while an arroba of wine was 16.133 liters (3.817 U.S. gallons).
a traditional Russian unit of distance. Peter the Great standardized the arshin at exactly 28 English inches, or 71.12 centimeters, early in the 1700s. The arshin was also used in several other countries adjacent to Russia. The arshin is also used as a unit of area equal to one square arshin; this would be equal to 5.4445 square feet or 0.5058 square meter.
a historic unit of volume, used for both liquid and dry measurement throughout the Middle East. In ancient times the artaba varied in size between about 35 and 55 liters. In recent centuries the Arab artaba, equal to about 66 liters, was a common unit in both Arab and non-Arab parts of the area.
a symbol used in astronomy for the arcsecond (see above).
as, ass, or aas
a traditional unit of mass for gold and silver, used in most of northern Europe but not in England. The unit varied in size from about 48 to 58 milligrams (0.75 to 0.90 English grain).
ASA number
for many years, the initials of the American Standards Association appeared on film packages in the United States as a measure of the speed of the photographic emulsion (the stuff on the film that "develops" to form the picture). The scale is arbitrary, but the important thing to know is that the speed at which the image registers on the film is proportional to the ASA number. Thus ASA 400 film registers an image twice as fast as ASA 200 film and four times as fast as ASA 100 film. The ASA number is now combined with the European DIN rating as a composite ISO rating. For example, ASA 400 film is now marked ISO 400/27°, because 27 is the DIN rating corresponding to ASA 400.
assay ton (AT)
a specialized unit of mass used by minerologists in assaying (testing) ores for the presence of gold, silver, platinum, uranium, or other valuable metals. One assay ton equals 29.1667 grams: just a little over an an ounce! The assay ton is actually a sample size. Since there are 87500/3 = 29166.7 troy ounces in a short ton (2000 lb), the number of milligrams of a precious metal in a sample of one assay ton is numerically equal to the number of troy ounces of that metal in one ton of raw ore. In Britain, the assay ton is based on the long ton and thus equals 98/3 = 32.6667 grams.
astronomical unit (ua or au or AU)
a unit of distance used by astronomers to measure distances in the Solar System. One astronomical unit equals the "average" distance from the center of the Earth to the center of the Sun (mathematically, it is the length of the semimajor axis of the Earth's elliptical orbit, which is the ordinary average of the Earth's minimum and maximum distances from the Sun). The currently accepted value, adopted in 2012, is exactly 149 597 870 700 meters (1.495 978 707 x 108 kilometers or about 92 955 807 miles), with an uncertainty of about 30 meters. The astronomical unit is a convenient yardstick for measuring the distances between objects in the Solar System. Astronomers find it particularly convenient to use astronomical units in solving the equations of planetary motion. Because these equations are the same regardless of the unit used for distance, the predictions they generate will remain correct even if future astronomers determine a slightly different length for the distance between the Earth and Sun. This unit is accepted for use with SI units. The official symbol for the unit is ua, but the symbol au is common in English-speaking countries. Note that AU is also used as the symbol for the absorbance unit (see above).
atmo-meter (atmo-m)
a unit used in atmospheric physics to compare the "depth" or total volume of atmospheres, or components of atmospheres. The depth (in atmo-meters) is equal to the depth (in meters) the atmosphere, or one gas component of the atmosphere, would have if it formed a uniform layer at standard temperature (0 °C) and pressure (1 atmosphere). One atmo-meter represents 2.686 99 x 1025 molecules of gas per square meter of planetary surface.
symbol for the technical atmosphere.
atmosphere (atm or atmos)
a unit of pressure designed to equal the average pressure of the Earth's atmosphere at sea level. In other pressure units, one atmosphere equals exactly 1013.25 millibars (mb), 101.325 kilopascals (kPa), approximately 29.92 inches of mercury (in Hg), 760.0 millimeters of mercury (mm Hg), or 14.6959 pounds of force per square inch (lbf/in2). This is the standard atmosphere; it equals 1.03323 technical atmosphere (that is, 1.03323 kilograms of force per square centimeter).
a medieval unit of time. The Greek word atomos means something "uncuttable," thus indivisible. In medieval times the Latin form atomus was also used to mean "a twinkling of the eye," the smallest amount of time imaginable. This was sometimes defined in a precise way equivalent to exactly 1/376 minute or about 160 milliseconds.
atomic mass unit (u or amu)
the unit of mass used by chemists and physicists for measuring the masses of atoms and molecules. Early in the nineteenth century, scientists discovered that each chemical element is composed of atoms, and that each chemical compound is composed of molecules in which atoms are combined in a fixed way. No one knew then just how small atoms and molecules really are, but as long as the relative weights of the different atoms were known, the outcome of chemical reactions could be predicted. These relative masses were determined by careful study of various reactions. The general idea was that atoms of hydrogen, known to be the lightest element, should have a mass of 1 amu, and all the other atoms should have masses which are multiples of this (then unknown) mass of the hydrogen atom. For a long time, physicists and chemists disagreed on the details of this definition. In 1960 they agreed on the definition of the unified atomic mass unit as 1/12 the mass of the most common atoms of carbon, known as carbon-12 atoms. (Most elements are mixtures of atoms which have different masses because they contain different numbers of neutrons; these varieties are called isotopes.) Careful experiments have measured the size of this unit; the currently accepted value (1998) is 1.660 538 782 x 10-27 kilograms. (This number equals 1 divided by Avogadro's number; see mole.) In addition, 1 amu equals approximately 931.494 028 MeV (see electron volt). In biochemistry, the atomic mass unit is called the dalton (Da).
atomic number
The atomic number was originally defined (about 1865) simply as an index describing the position of an element in the periodic table. Not until 1913 was it known that the atomic number is actually a unit of measurement, equal to the number of electrons surrounding a neutral (uncharged) atom, and also to the number of protons in the nucleus.
atto- (a-)
a metric prefix denoting 10-18 (one quintillionth). For example, one electron volt equals 0.1602 attojoule. The root of the prefix is atten, the word for 18 in Danish and Norwegian.
AU or au
in astronomy, the synbol for the astronomical unit; in medicine, a symbol for allergy units.
abbreviation for the Latin phrase ab urbe condita, "from the founding of the city." The Romans counted years from the legendary founding of Rome by Romulus and Remus, an event placed in 753 BC (or BCE) in our present calendar. The Christian Church continued to count years AUC for centuries after the fall of Rome. Our year 2002 is 2755 AUC.
aught (/0)
a unit used by jewelers and craftspeople to measure the size of small beads (often called seed beads). The measurement scale is inverted: larger numbers of aughts correspond to smaller beads. Beads of size 11/0, a common size, average a little less than 2 millimeters in diameter, and other sizes are more-or-less inversely proportional. The measure may have originated as the number of beads that could comfortably be strung on one inch of cord; with present sizes a string of n beads of size n/0 occupies about 0.8 inch (20 mm). The word aught, meaning zero, is a fairly recent corruption of the old English word naught, meaning nothing; apparently the phrase a naught, meaning a zero, came to be misspelled as an aught.
an old English wine measure equal to about 40 gallons (roughly 150 liters). The aume is the English version of a German unit, the ohm [2].
a traditional French unit of distance, varying from region to region but similar in length to the English ell.
av or avdp
abbreviations for avoirdupois (see below).
Avogadro's number, Avogadro constant (NA)
a unit of relative quantity equal to the number of atoms or molecules per mole of a substance. The currently accepted value is 6.022 141 79 x 1023 per mole with an uncertainty of 0.000 000 47 x 1023 per mole (about 80 parts per billion). The atomic mass unit (see above), in grams, is equal to one divided by this number. The unit is named for the Italian chemist and physicist Amedeo Avogadro (1776-1856). Avogadro was the first to conclude from Dalton's atomic theory that equal volumes of gases (at the same temperature and pressure) must contain an equal number of molecules.

Amadeo Carlo Avogadro (1776-1856)
Italian chemist and physicist
avoirdupois weights
the common traditional system of weights in all the English-speaking countries. Until the introduction of the metric system, almost all weights were stated in avoirdupois units, with only precious metals being measured by troy weights and pharmaceuticals by apothecary weights (see above). The name of the system comes from the Old French phrase avoir du pois or aveir de pois, "goods of weight," indicating simply that the goods were being sold by weight rather than by volume or by the piece. The system is based on the avoirdupois pound [1] of 7000 grains. The pound is divided into 16 ounces [1], each divided further into 16 drams [1]. The avoirdupois system was introduced in England around 1300, replacing an older commercial system based on a "mercantile pound" (libra mercatoria) of 7200 grains divided into exactly 15 troy ounces [2]. Scholars believe the avoirdupois pound was invented by wool merchants and modeled on a pound of 16 ounces used in Florence, Italy, which was an important buyer of English wool at the time. The avoirdupois weights quickly became the standard weights of trade and commerce. They continue to be used for most items of retail trade in the United States, and they remain in some use in Britain, Canada, and other areas of British heritage despite the introduction of metric units there.
abbreviation for American Wire Gauge. See gauge [3] and the table of wire gauge equivalents.

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March 21, 2001; revised May 4, 2018.