The Collaborative International Dictionary of English v.0.48:

Equivalent \E*quiv"a*lent\ ([-e]*kw[i^]v"[.a]*lent), n.
   1. Something equivalent; that which is equal in value, worth,
      weight, or force; as, to offer an equivalent for damage
      done.
      [1913 Webster]

            He owned that, if the Test Act were repealed, the
            Protestants were entitled to some equivalent. . . .
            During some weeks the word equivalent, then lately
            imported from France, was in the mouths of all the
            coffeehouse orators.                  --Macaulay.
      [1913 Webster]

   2. (Chem.) That comparative quantity by weight of an element
      which possesses the same chemical value as other elements,
      as determined by actual experiment and reference to the
      same standard. Specifically:
      (a) The comparative proportions by which one element
          replaces another in any particular compound; thus, as
          zinc replaces hydrogen in hydrochloric acid, their
          equivalents are 32.5 and 1.
      (b) The combining proportion by weight of a substance, or
          the number expressing this proportion, in any
          particular compound; as, the equivalents of hydrogen
          and oxygen in water are respectively 1 and 8, and in
          hydric dioxide 1 and 16.
          [1913 Webster]

   Note: This term was adopted by Wollaston to avoid using the
         conjectural expression atomic weight, with which,
         however, for a time it was practically synonymous. The
         attempt to limit the term to the meaning of a
         universally comparative combining weight failed,
         because of the possibility of several compounds of the
         substances by reason of the variation in combining
         power which most elements exhibit. The equivalent was
         really identical with, or a multiple of submultiple of,
         the atomic weight.
         [1913 Webster]

   3. (Chem.) A combining unit, whether an atom, a radical, or a
      molecule; as, in acid salt two or more equivalents of acid
      unite with one or more equivalents of base.
      [1913 Webster]

   Mechanical equivalent of heat (Physics), originally defined
      as the number of units of work which the unit of heat can
      perform, equivalent to the mechanical energy which must be
      expended to raise the temperature of a pound of water one
      degree Fahrenheit; later this value was defined as one
      British thermal unit (B.t.u). Its value was found by
      Joule to be 772 foot pounds; later measurements give the
      value as 777.65 foot-pounds, equivalent to 107.5
      kg-meters. This value was originally called Joule's
      equivalent, but the modern Joule is defined differently,
      being 10^7 ergs. The B.t.u. is now given as 1,054.35
      absolute Joules, and therefore 1 calorie (the amount of
      heat needed to raise one gram of water one degree
      centigrade) is equivalent to 4.186 Joules.
      [1913 Webster + PJC]

   Note: The original definition of the Mechanical equivalent of
         heat in the 1913 Webster was as below. The difference
         between foot pounds and kilogram-meters ("on the
         centigrade scale") is puzzling as it should be a factor
         of 7.23, and the figure given for kilogram-meters may
         be a mistaken misinterpretation of the report. -- PJC:
         The number of units of work which the unit of heat can
         perform; the mechanical energy which must be expended
         to raise the temperature of a unit weight of water from
         0[deg] C. to 1[deg] C., or from 32[deg] F. to 33[deg]
         F. The term was introduced by Dr. Mayer of Heilbronn.
         Its value was found by Joule to be 1390 foot pounds
         upon the Centigrade, or 772 foot pounds upon the
         Fahrenheit, thermometric scale, whence it is often
         called Joule's equivalent, and represented by the
         symbol J. This is equal to 424 kilogram meters
         (Centigrade scale). A more recent determination by
         Professor Rowland gives the value 426.9 kilogram
         meters, for the latitude of Baltimore.
         [1913 Webster +PJC]

WordNet (r) 3.0 (2006):

British thermal unit
    n 1: a unit of heat equal to the amount of heat required to
         raise one pound of water one degree Fahrenheit at one
         atmosphere pressure; equivalent to 251.997 calories [syn:
         British thermal unit, BTU, B.Th.U.]