The Collaborative International Dictionary of English v.0.48:

Unit \U"nit\, n. [Abbrev. from unity.]
   1. A single thing or person.
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   2. (Arith.) The least whole number; one.
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            Units are the integral parts of any large number.
                                                  --I. Watts.
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   3. A gold coin of the reign of James I., of the value of
      twenty shillings. --Camden.
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   4. Any determinate amount or quantity (as of length, time,
      heat, value) adopted as a standard of measurement for
      other amounts or quantities of the same kind.
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   5. (Math.) A single thing, as a magnitude or number, regarded
      as an undivided whole.
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   Abstract unit, the unit of numeration; one taken in the
      abstract; the number represented by 1. The term is used in
      distinction from concrete, or determinate, unit, that is,
      a unit in which the kind of thing is expressed; a unit of
      measure or value; as 1 foot, 1 dollar, 1 pound, and the
      like.

   Complex unit (Theory of Numbers), an imaginary number of
      the form a + broot-1, when a^2 + b^2 = 1.

   Duodecimal unit, a unit in the scale of numbers increasing
      or decreasing by twelves.

   Fractional unit, the unit of a fraction; the reciprocal of
      the denominator; thus, 1/4 is the unit of the fraction
      3/4.

   Integral unit, the unit of integral numbers, or 1.

   Physical unit, a value or magnitude conventionally adopted
      as a unit or standard in physical measurements. The
      various physical units are usually based on given units of
      length, mass, and time, and on the density or other
      properties of some substance, for example, water. See
      Dyne, Erg, Farad, Ohm, Poundal, etc.

   Unit deme (Biol.), a unit of the inferior order or orders
      of individuality.

   Unit jar (Elec.), a small, insulated Leyden jar, placed
      between the electrical machine and a larger jar or
      battery, so as to announce, by its repeated discharges,
      the amount of electricity passed into the larger jar.

   Unit of heat (Physics), a determinate quantity of heat
      adopted as a unit of measure; a thermal unit (see under
      Thermal). Water is the substance generally employed, the
      unit being one gram or one pound, and the temperature
      interval one degree of the Centigrade or Fahrenheit scale.
      When referred to the gram, it is called the gram degree.
      The British unit of heat, or thermal unit, used by
      engineers in England and in the United States, is the
      quantity of heat necessary to raise one pound of pure
      water at and near its temperature of greatest density
      (39.1[deg] Fahr.) through one degree of the Fahrenheit
      scale. --Rankine.

   Unit of illumination, the light of a sperm candle burning
      120 grains per hour. Standard gas, burning at the rate of
      five cubic feet per hour, must have an illuminating power
      equal to that of fourteen such candles.

   Unit of measure (as of length, surface, volume, dry
      measure, liquid measure, money, weight, time, and the
      like), in general, a determinate quantity or magnitude of
      the kind designated, taken as a standard of comparison for
      others of the same kind, in assigning to them numerical
      values, as 1 foot, 1 yard, 1 mile, 1 square foot, 1 square
      yard, 1 cubic foot, 1 peck, 1 bushel, 1 gallon, 1 cent, 1
      ounce, 1 pound, 1 hour, and the like; more specifically,
      the fundamental unit adopted in any system of weights,
      measures, or money, by which its several denominations are
      regulated, and which is itself defined by comparison with
      some known magnitude, either natural or empirical, as, in
      the United States, the dollar for money, the pound
      avoirdupois for weight, the yard for length, the gallon of
      8.3389 pounds avoirdupois of water at 39.8[deg] Fahr.
      (about 231 cubic inches) for liquid measure, etc.; in
      Great Britain, the pound sterling, the pound troy, the
      yard, or 1/108719 part of the length of a second's
      pendulum at London, the gallon of 277.274 cubic inches,
      etc.; in the metric system, the meter, the liter, the
      gram, etc.

   Unit of power. (Mach.) See Horse power.

   Unit of resistance. (Elec.) See Resistance, n., 4, and
      Ohm.

   Unit of work (Physics), the amount of work done by a unit
      force acting through a unit distance, or the amount
      required to lift a unit weight through a unit distance
      against gravitation. See Erg, Foot Pound,
      Kilogrammeter.

   Unit stress (Mech. Physics), stress per unit of area;
      intensity of stress. It is expressed in ounces, pounds,
      tons, etc., per square inch, square foot, or square yard,
      etc., or in atmospheres, or inches of mercury or water, or
      the like.
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