unit

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun An individual, group, structure, or other entity regarded as an elementary structural or functional constituent of a whole.
• noun A group regarded as a distinct entity within a larger group.
• noun A mechanical part or module.
• noun An entire apparatus or the equipment that performs a specific function.
• noun A precisely specified quantity in terms of which the magnitudes of other quantities of the same kind can be stated.
• noun Medicine The quantity of a vaccine, serum, drug, or other agent necessary to produce a specific effect.
• noun A fixed amount of scholastic study used as a basis for calculating academic credits, usually measured in hours of classroom instruction or laboratory work.
• noun A section of an academic course focusing on a selected theme.
• noun The number immediately to the left of the decimal point in the Arabic numeral system.
• noun The lowest positive whole number; one.
• noun An element of a ring with a multiplicative inverse.

from The Century Dictionary.

• noun A single thing or person, opposed to a plurality; also, any group regarded as individual in a plurality of similar groups; any one of the individuals or similar groups into which a complex whole may be analyzed.
• noun Any standard quantity by the repetition and subdivision of which any other quantity of the same kind is measured.
• noun Any subdivision of an army having a distinct organization and defined duties.
• noun According to the hypothesis of Herbert Spencer, an ultimate biological unit or element which, when joined to others like itself, possesses the power to become a specific organism. The body of each individual organism is held to consist of its own sort of physiological units which are all alike and nearly, but not completely, identical with those which compose the body of another individual of the same species. The physiological unit is held to be intermediate between the molecule or chemical unit and the cell or morphological unit, each cell being regarded as composed of innumerable physiological units each of which again consists of innumerable molecules. The physiological units are held to make each organism and each species what it is and to have the aptitude to contribute to the construction of the organism in virtue of their polarity. The hypothesis of physiological units is advanced as an explanation of the facts of inheritance in general, and, especially, the generation of living beings from eggs and the regeneration or replacement of lost parts. The organism is able to replace lost parts because the polarity of the units, it is said, causes them to restore the organism to its perfect condition under the directive influence of the whole, which forces the units to arrange themselves in just such a way as is necessary for the perfection of the part in the harmony of the whole. A germ-cell is held to contain small groups of these units which, by their polarity, give to it the power to reproduce the whole.
• noun According to Bateson, an ultimate element or unit of inheritance, of unknown nature, of which an allelomorph or character-unit is the sensible manifestation.
• noun In the centesimal method, a grade.
• noun Acre-foot, a unit used in irrigation; the volume of water required to cover one acre to a depth of one foot, = 43,560 cubic feet, = 1,233.49 cubic meters.
• noun Ampere, a practical unit of electrical current, = 0.1 c. g. s. unit.
• noun Ampere (international), the ampere as defined for practical purposes by the International Congress at Chicago in 1893, as the current required to deposit 0.001118 gram of silver in a second of time. This value was subsequently legalized in the United States and was re-adopted by the Electrical Conference in England (1908), although the value 0.0011183 was known to be more nearly correct.
• noun Ampere-hour, =3,600 coulombs, =360 c. g. s. units (electromagnetic).
• noun Ampere-second, =1 coulomb.
• noun Ampere-turn, a unit of magnetomotive force, =4 π/10 c. g. s. units, =1.256637 gilberts.
• noun Angström unit, =0.0000001 millimeter, =0.0001 micron.
• noun Arc, = 100 square meters, =1,076.387 square feet.
• noun Atmosphere, a unit of fluid pressure, = the pressure of a column of mercury 76 centimeters in height, =1,013,240 dynes per square centimeter. Sometimes an atmosphere is defined as a pressure of 1 kilogram per square centimeter.
• noun Bougie décimale, a unit of intensity of light originally defined as 1/20 of a violle; subsequently (by the Geneva Congress of Photometricians), as equal to 1 hefner.
• noun Bougie de l'étoile, a unit of intensity of light used in France; the light from a stearin candle burning 10 grams per hour, = 1/7 carcels (approximately).
• noun British thermal unit, the heat required to raise one pound (avoir.) of water one degree Fahrenheit, = 1,054.90 joules, = 251.996 calories, = 778.104 foot-pounds, = 0.000392982 horse-power hour, = 0.293027 watt-hour.
• noun Calory, a calorimetric unit; the heat required to raise one gram of water one degree centigrade (also called a gram-calory or small calory), =4.18617 joules, = 3.08777 foot-pounds, =0.003968 British thermal unit, = 0.00116282 watt-hour.
• noun Candle (British standard), a unit of intensity of light; the light from a spermaceti candle of specified composition, size, and form (see standard candle), = 1.136 Hefner units.
• noun Candle (German; German Vereinskerze), a unit of intensity of light; the light from a paraffin candle having a diameter of 20 millimeters and a flame height of 50 millimeters, =1.16 … 1.224 Hefner units (approximately), =1.05 British standard candles (approximately).
• noun Candle (Munich), a unit of intensity of light; the light from a stearin candle of conical form, =0.153 carcel (approximately), =1.17. British standard candles (approximately).
• noun Candle (star). See bougie de l'étoile (above).
• noun Candle-foot. See foot-candle (below).
• noun Candle-meter. See meter-candle (below).
• noun Carcel, a unit of intensity of light formerly used in France; the light from a lamp of Argand type with mechanical draft, =10.87 … 10.90 Hefner units (approximately), =9.53 British candles (approximately).
• noun Centimeter, =0.01 meter, =0.393700 inch.
• noun Centimeter (cubic), =0.001 liter, =0.0610234 cubic inch.
• noun C. G. S. unit of acceleration, an acceleration of one centimeter per second per second, =0.00101979 of the acceleration due to gravity.
• noun C. G. S. unit of capacity (electromagnetic), the capacity of a condenser the charge of which at unit potential (c. g. s.) is one c. g. s. unit of quantity or 10 coulombs, = 1 × 109 farads.
• noun C. G. S. unit of capacity (electrostatic), =1/9 × 10–20 c. g. s. units of capacity of the electromagnetic system (approximately), =1/9 × 10–11 farads (approximately).
• noun C. G. S. unit of electric charge (electromagnetic), the charge transferred by one c. g. s. unit of current in one second of time, =10 coulombs, =0.00277778 ampere-hour, = 3 × 1010 electrostatic units (approximately).
• noun C. G. S. unit of electric charge (electrostatic), the charge which in air exerts a force of one dyne on an equal charge at a distance of one centimeter, =1/v c. g. s. units (electromagnetic), =10/V coulombs or 1/3 × 10coulombs (approximately).
• noun C. G. S. unit of electric current, the current which, flowing in a circular coil of one centimeter radius, produces at the center of the coil a magnetic field of 2μ units intensity, =10 amperes.
• noun C. G. S. unit of electrical resistance, =1 × 10–9 ohms.
• noun C. G. S. unit of electromotive force, =1 × 10–8 volts.