Source : Webster's Revised Unabridged Dictionary (1913)

Equivalent \E*quiv"a*lent\, n.
   1. Something equivalent; that which is equal in value, worth,
      weight, or force; as, to offer an equivalent for damage
      done.

            He owned that, if the Test Act were repealed, the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.                          --Macaulay.

   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.

Equivalent \E*quiv"a*lent\, a. [L. aequivalens, -entis, p. pr.
   of aequivalere to have equal power; aequus equal + valere to
   be strong, be worth: cf. F. ['e]quivalent. See {Equal}, and
   {Valiant}.]
   1. Equal in wortir or value, force, power, effect, import,
      and the like; alike in significance and value; of the same
      import or meaning.

            For now to serve and to minister, servile and
            ministerial, are terms equivalent.    --South.

   2. (Geom.) Equal in measure but not admitting of
      superposition; -- applied to magnitudes; as, a square may
      be equivalent to a triangle.

Equivalent \E*quiv"a*lent\, v. t.
   To make the equivalent to; to equal; equivalence. [R.]

Source : WordNet®

equivalent
     adj 1: equal in amount or value; "like amounts"; "equivalent
            amounts"; "the same amount"; "gave one six blows and
            the other a like number"; "an equal number"; "the same
            number" [syn: {like}, {equal}, {same}] [ant: {unlike}]
     2: being essentially equal to something; "it was as good as
        gold"; "a wish that was equivalent to a command"; "his
        statement was tantamount to an admission of guilt" [syn: {tantamount(p)}]

equivalent
     n 1: a person or thing equal to another in value or measure or
          force or effect or significance etc; "send two dollars
          or the equivalent in stamps"
     2: the atomic weight of an element that has the same combining
        capacity as a given weight of another element; the
        standard is 8 for oxygen [syn: {equivalent weight}, {combining
        weight}, {eq}]