N

N: 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.
nail: 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. See Prefix
nanoacre: a humorous unit of area on a computer chip, equal to one billionth of an acre or about 4.047 square millimeters.
nanocurie (nCi): a common unit of radioactivity. The nanocurie equals 10^-9curie 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.
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.
nanon: an informal name for the nanometer.
nanosecond (ns): a unit of time equal to 10^-9 second.
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.
nautical mile (nmi, naut mi or NM): a unit of distance used primarily at sea. 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 (1853.18 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 (1853.24 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. See Length
nebuchadnezzar: a huge wine bottle holding about 15 liters, 20 times the volume of a regular bottle.
neper (Np): a unit expressing the ratio of two numbers as a natural logarithm; the ratio r is equal 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 British 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).
nest: an old English unit of quantity equal to 3. This unit was often, but not always, used for items which 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 7.23301 poundals. The newton is also equal to about 0.101972 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. See Force
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 0.7376 pound foot. See Torque
ngarn: a unit of land area in Thailand. The ngarn equals 1/4 rai, 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.
nibble: 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.
nile: 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.
nip: an informal unit of liquid volume. The term "nip" usually means "a small amount," with no precise equivalent. In Britain, however, a nip of spirits is considered to be 1/6 gill (about 22.95 milliliters or 0.776 U.S. fluid ounce).
nit (nt): an MKS unit of luminance, equal to one candela per square meter, or 10^-4stilb.
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.
noggin: 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).
nonet: a unit of quantity equal to 9, used mostly in music to describe an ensemble of 9 instruments.
nook: a traditional unit of land area in Scotland and northern England. A nook of land is 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.
note: a unit of relative time in music, also called a whole note, equal to 1/2 breve.
nox: 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."
noy: 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.
NTU: abbreviation for nephelometric turbitity unit, a unit used in measuring water quality. Turbidity is an optical property: the scattering and absorbtion of light by solids suspended in water. In other words, water is turbid if you can't see through it. 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 of a synthetic polymer called formazin. Drinking water should not have a turbidity above 1 NTU, although values up to 5 NTU are usually considered safe.
nu: 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.
Nw: abbrevation 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.
nybble: see nibble, above.