a unit of refractive index sometimes used in atmospheric science. The index of refraction of the atmosphere is only slightly greater than 1. The value of the index in N units is the number of millionths by which the index exceeds one; that is, an index of refraction n is equivalent to (n - 1) x 106 N units.
an old English unit of length equal to 1/20 ell. Like the ell, the nail was used for measuring cloth; traditionally, it represented the length of the last two joints (including the fingernail) of the middle finger. The nail is equivalent to 1/16 yard, 1/4 span, 2.25 inches, or exactly 5.715 centimeters.
nano- (n-)
a metric prefix meaning 10-9, or one billionth. The prefix comes from the Greek word nanos, dwarf.
a humorous unit of area on a computer chip, equal to one billionth of an acre or about 4.047 square millimeters.
nanobar (nb or nbar)
a CGS unit of pressure equal to 10-9 bar or 0.1 millipascal (mPa). The nanobar is frequently used in meteorology to express the partial pressure of atmospheric ozone.
nanocurie (nCi)
a common unit of radioactivity. The nanocurie equals 10-9 curie or 37 becquerels; this corresponds to a radioactivity of 37 atomic disintegrations per second.
nanofarad (nF)
a common metric unit of electric capacitance equal to 10-9 farad. This unit was previously called the millimicrofarad (mµF).
nanogram (ng)
a metric unit of mass equal to 10-9 gram, or one millionth of a milligram.
nanogray (nGy)
a unit of radiation dose equal to 10-9 gray or 0.1 microrad (µrad). This unit often occurs in the study of inhalation exposures.
nanoliter (nl or nL)
a metric unit of volume equal to 10-9 liter or 0.001 cubic millimeter.
nanometer (nm)
a metric unit of distance equal to 10-9 meter. Introduced in 1951, the nanometer replaced the millimicron. One nanometer equals 0.001 micrometer or 10 angstroms.
a unit of amount of substance equal to 10-9 mole. This unit is common in biochemistry, since a mole of a large organic molecule can be quite a large amount.
an informal name for the nanometer.
nanonewton (nN)
a metric unit of force equal to 10-9 newton (see below) or 0.1 millidyne. Nanonewtons measure the force of solar radiation and the tiny forces exerted within living cells.
nanoradian (nrad)
a unit of angle measure equal to 10-9 radian. The nanoradian equals about 0.208 533 milliarcsecond (mas). Such tiny angles are encountered in astronomy and in geological measurements.
nanosecond (ns)
a unit of time equal to 10-9 second.
nanosievert (nSv)
a unit of radiation dose equal to 10-9 sievert or 0.1 microrem.
nanostrain (nstrain)
an engineering unit measuring strain. An object under strain is typically deformed (extended or compressed), and the strain is measured by the amount of this deformation relative to the same object in an undeformed state. One nanostrain is the strain producing a deformation of one part per billion (10-9). Strains in geological formations are often measured in this unit.
nanotesla (nT)
a unit of magnetic field strength equal to 10-9 tesla or 10-5 gauss. The unit is used in geology to measure small changes in the Earth's magnetic field.
nat or natural unit
a unit of information content used in information and communications theory. The nat is similar to the shannon but uses the natural logarithm (to the base e) instead of the logarithm to the base 2. If the probability of receiving a particular message is p, then the information content of the message is -loge p nats. For example, if a message is a string of 5 letters or numerals, with all combinations being equally likely, then a particular message has probability 1/365 and the information content of a message is 5(loge 36) = 17.9176 nats. One nat equals log2 e = 1.442 695 shannons or log10 e = 0.434 294 hartleys.
nautical mile (nmi, naut mi, n mile, or NM)
a unit of distance used primarily at sea and in aviation. The nautical mile is defined to be the average distance on the Earth's surface represented by one minute of latitude. This may seem odd to landlubbers, but it makes good sense at sea, where there are no mile markers but latitude can be measured. Because the Earth is not a perfect sphere, it is not easy to measure the length of the nautical mile in terms of the statute mile used on land. For many years the British set the nautical mile at 6080 feet (1853.188 meters), exactly 800 feet longer than a statute mile; this unit was called the Admiralty mile. Until 1954 the U.S. nautical mile was equal to 6080.20 feet (1853.249 meters). In 1929 an international conference in Monaco redefined the nautical mile to be exactly 1852 meters or 6076.115 49 feet, a distance known as the international nautical mile. The international nautical mile equals about 1.1508 statute miles. There are usually 3 nautical miles in a league. The unit is designed to equal 1/60 degree [2], although actual degrees of latitude vary from about 59.7 to 60.3 nautical miles. (Note: using data from the Geodetic Reference System 1980, the "true" length of a nautical mile would be 1852.216 meters.)
a symbol for the newton centimeter, a metric unit of torque equal to 0.01 newton meter (see below) or about 1.416 12 inch ounces (in·oz) in traditional English units.
a symbol sometimes used for the normal liter (see below). Recall that a liter is the same as a cubic decimeter (dm3).
a huge wine bottle holding about 15 liters, 20 times the volume of a regular bottle.
neck (nk)
an informal unit of distance used to measure the distance one horse leads another at the finish of a race. The neck is usually interpreted to be 1/4 length or a little less; this is roughly 2 feet or 0.6 meter.
neper (Np)
a unit expressing the ratio of two numbers as a natural logarithm; the ratio r corresponds to (1/2) ln r nepers. Quantities differ by 1 neper if one is e2 = 7.389056 times the other. One neper is equal to about 8.685 890 decibels, and in general n nepers equal 20n/(ln 10) decibels. The neper is accepted for use with SI units, and it may become an SI unit in the future. The unit recognizes the Scottish mathematician John Napier (1550-1617), the inventor of the logarithm (Napier often spelled his name Jhone Neper, and he used the Latin form Ioanne Napero in his writings).
an old English unit of quantity equal to 3. This unit was often, but not always, used for items that nest together, such as 3 bowls or 3 hampers.
net ton (NT)
the name "net ton" is used in at least two ways: (1) as another name for the U.S. or short ton of 2000 pounds (see ton [1]), and (2) as another name for the register ton, a unit of volume equal to 100 cubic feet (see ton [3]), in describing the cargo-carrying capacity of a ship as opposed to the entire interior volume. To avoid confusion, it is better to use "short ton" for use (1) and "net register ton" for use (2).
new candle
an older name (introduced in 1937) for the candela, a unit of light intensity.
new style (NS)
a notation used after dates to indicate that the date is stated in the Gregorian calendar (the calendar now in general use) rather than in the Julian calendar (see year [2]). The notation is used primarily for Gregorian dates between 15 October 1582, when the Gregorian calendar was adopted in Catholic Europe, and 14 September 1752, when it was adopted in Britain. See also old style.
newton (N)
the SI unit of force. A force of one newton will accelerate a mass of one kilogram at the rate of one meter per second per second. The corresponding unit in the CGS system is the dyne; there are 105 dynes in one newton. In traditional English terms, one newton is about 0.224 809 pounds of force (lbf) or 7.233 01 poundals. The newton is also equal to about 0.101 972 kilograms of force (kgf) or kiloponds (kp). The newton is named for Isaac Newton (1642-1727), the British mathematician, physicist, and natural philosopher. He was the first person to understand clearly the relationship between force (F), mass (m), and acceleration (a) expressed by the formula F = ma.
newton meter (N·m)
the SI unit of torque. Torque, the tendency of a force to cause a rotation, is the product of the force and the distance from the center of rotation to the point where the force is applied. Torque has the same units as work or energy, but it is a different physical concept. To stress the difference, scientists measure torque in newton meters rather than in joules, the SI unit of work. One newton meter is approximately 0.737 562 pound foot.
newton per coulomb (N/C)
the SI unit of electric field strength. An electric charge creates an electric field that attracts (or repels) other charges with a force proportional to the size of the charge. Since the volt is defined as a joule per coulomb, and the joule is equivalent to a newton meter, the newton per coulomb is equivalent to the volt per meter.
newton per meter (N/m)
the SI unit of surface tension. The molecules at the surface of a fluid have a higher average energy than those in the interior because they are less constrained by neighboring molecules. The tendency of the surface to contract to minimize this surface energy is what we call surface tension; it is measured by the rate of change in energy per unit of surface area. The corresponding SI unit would be the joule per square meter. Since the joule is equivalent to a newton meter, the joule per square meter is equivalent to the newton per meter. In practice surface tension is typically measured in millinewtons per meter (mN/m). It happens that one millinewton per meter is equal to one dyne per centimeter (dyn/cm), which is the CGS unit of surface tension.
newton second (N·s)
the SI unit of impulse, equal to the amount of momentum added to an object if a force of one newton is applied for one second. The newton second equals about 0.224 809 pound second (lbf·s) in traditional English units. Impulse and momentum have the same dimensions, but momentum is measured in kilogram meters per second (kg·m/s) in the SI.
a unit of land area in Thailand. The ngarn equals 1/4 rai, 100 talangwah (tw), or exactly 400 square meters (478.396 square yards or just under 0.1 acre).
niacin equivalent (NE)
a unit used in nutrition. An essential nutrient, niacin is supplied in normal diets from tryptophan, an amino acid found in many foods. However, only a small fraction of tryptophan is converted into niacin in the body. Accordingly, 1 niacin equivalent is equal to 1 milligram of actual niacin or to 60 milligrams of tryptophan.
a unit of information used in computer science. A nibble is 4 bits or 1/2 byte. The cuter spelling nybble, suggested by byte, is sometimes used. In different contexts, a group of 4 bits is sometimes called a quadbit or a hexit.
a unit used in nuclear engineering to describe the "reactivity" of a nuclear reactor. One nile is a reactivity of 0.01 or 1%. Since this is a rather large amount, reactivity is measured for practical purposes in milliniles, with one millinile = 10-5. The nile was invented in the British nuclear power industry. For a discussion of reactivity, see inhour.
nine [1]
a measure of "fineness" or purity of gold and other materials. For example, gold that is 99.99% pure, or .9999 fine, is called "4 nines fine."
nine [2]
a measure of reliability, or "availability," used in computer engineering. For example, a component has "3 nines" of reliability if it operates correctly 99.9% of the time, or, equivalently, if its failure rate is 0.1%. Thus each nine represents a reduction of 90% in the failure rate. If the probability of correct operation is p, then the reliability, in nines, is equal to -log10(1-p).
an informal unit of liquid volume. The term "nip" often means "a small amount," with no precise equivalent. In U.S. bartending, a nip is often taken to be 2 fluid ounces (about 59 milliliters). In Britain, a nip of spirits is considered to be 1/6 gill (about 22.95 milliliters or 0.776 U.S. fluid ounce); a nip of beer is 1/4 pint (the same as a gill, 4 fluid ounces or about 117.7 milliliters) or sometimes 1/3 pint (189.4 milliliters).
nit (nt)
an MKS unit of luminance, equal to one candela per square meter, or 10-4 stilb. The nit is not approved as part of the SI, but it has been approved since 1947 by the International Commission on Illumination (CIE) and is in wide use. The name of the unit comes from the Latin niteo, to shine. This unit is sometimes called the meterlambert, by analogy with the footlambert.
Nm or N·m [1]
symbol for newton meter, a unit of torque (see above).
Nm [2]
abbrevation for normal metric, a symbol for the density of worsted yarns. Yarn is described as a/b Nm if it is a-ply (has a individual strands) and there are b kilometers of yarn per kilogram. An example might be 2/40 Nw, describing a 2-ply yarn having 40 kilometers per kilogram. If the yarn is single-ply, the a is often omitted, so a single-ply yarn is described as b Nm.
the French word for the knot (1.852 km/h), the traditional unit of velocity at sea. This unit is still used officially in France; for example, Météo France uses it in maritime weather forecasts.
a traditional unit of liquid volume, used primarily in Ireland. Like the gill, the noggin is often taken to equal 1/4 pint; sometimes it is taken to be 1/2 pint.
noise criterion (NC), noise rating (NR)
units used in enginering to measure the acceptability of sound levels in enclosed spaces. The noise rating (NR) system was introduced by Kosten and van Os in 1962. Numerically, the NR rating is equal to the sound level in decibels at a frequency of 1000 hertz, but the rating requires lower levels at the more objectionable higher frequencies. The noise criterion (NC) system, introduced by Beranek in 1957, is similar, but it is designed more toward preserving speech communication, requiring lower levels of both high and low frequencies. It was updated in 1971, and the updated system is called PNC (prefered noise criterion).
a unit of quantity equal to 9.
a unit of quantity equal to 9, used mostly in music to describe an ensemble of 9 instruments.
a traditional unit of land area in northern England. Originally the nook was 1/2 virgate; a virgate, often called a yardland in the north, was about 30 acres in southern England but tended to be closer to 40 acres in the north. A nook of land thus came to be 20 acres or about 8.094 hectares.
normal (N)
a term used in chemistry to describe a solution having a concentration of 1 gram equivalent per liter. The normal concentration of an ion is effectively equal to the molar concentration divided by the valence (the number of free or missing electrons) of the ion.
normal cubic meter (Nm3)
a unit of mass for gases equal to the mass of 1 cubic meter (35.3147 ft3) at a pressure of 1 atmosphere and at a standard temperature, often 0 °C (32 °F) or 20 °C (68 °F). The term standard cubic meter is also used; sometimes both are used, with different temperatures. Because industry practice varies, these terms should always be defined wherever they occur. The symbol Nm3, though common, is not permissible under the SI because that symbol means "newton cubic meter." An acceptable SI symbol would be "m3 normal." See also sccm.
normal liter (NL or Nl or Ndm3)
a unit of mass for gases equal to the mass of 1 liter (0.035 3147 ft3) at a pressure of 1 atmosphere and at a standard temperature, often 0 °C (32 °F) or 20 °C (68 °F). Air flow is often stated in normal liters per minute (Nl/min).
an informal unit of length, equal to roughly half the length of a horse's head, used in expressing the results of a horse race.
a unit of relative time in music, also called a whole note, equal to 1/2 breve.
a unit of (low) illumination equal to 0.001 lux. This unit was used in Germany during World War II to describe permitted levels of lighting during air raids. "Nox" is the Latin word for "night."
a unit of perceived noisiness introduced by the American acoustics engineer K. D. Kryter in 1959. An observer uses the unit to described the noise levels he or she experiences as multiples of a standard level. The standard level, one noy, is defined to be the noisiness of a random noise signal within the frequency band from 910 to 1090 hertz at a sound pressure level of 40 decibels. As you have already guessed, the unit's name is chosen so that its plural is pronounced "noise." A jet aircraft takeoff is rated at about 110 noys.
abbreviation for nephelometric turbidity unit, a unit used in measuring water quality. Turbidity is an optical property: the scattering and absorption of light by solids suspended in water. Turbid water has a cloudy or hazy appearance. An instrument called a nephelometer (from a Greek word meaning "cloudy") measures turbidity directly by comparing the amount of light transmitted straight through a water sample with the amount scattered at an angle of 90° to one side; the ratio determines the turbidity in NTU's. The instrument is calibrated using samples of a standard solution such as formazin, a synthetic polymer. Drinking water should not have a turbidity above 1 NTU, although values up to 5 NTU are usually considered safe. Outside the U.S., this unit is usually called the FNU (formazin nephelometric unit).
a measure of the dispersiveness (or constringence, as it is called) of a lens or prism. If b, y, and r are the indices of refraction at blue, yellow, and red wavelengths, respectively, then the nu value is (y - 1)/(b - r). The measure is represented by the Greek letter nu, and sometimes the symbol "nu" is used as if it were a unit, as in 65 nu.
symbol for normal worsted, a unit measuring the density of worsted yarns. Worsted yarn is described as a/b Nw if it is a-ply (has a individual strands) and there are b 560-yard hanks of yarn per pound. An example might be 2/40 Nw, describing a 2-ply yarn having 40 hanks (22 400 yards) per pound. If the yarn is single-ply, the a is often omitted, so a single-ply worsted yarn is described as b Nw.
see nibble, above.

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Checked and revised December 17, 2008