The Collaborative International Dictionary of English v.0.48:

Equivalent \E*quiv"a*lent\ ([-e]*kw[i^]v"[.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 worth or value, force, power, effect, import, and
      the like; alike in significance and value; of the same
      import or meaning.
      [1913 Webster]

            For now to serve and to minister, servile and
            ministerial, are terms equivalent.    --South.
      [1913 Webster]

   2. (Geom.) Equal in measure but not admitting of
      superposition; -- applied to magnitudes; as, a square may
      be equivalent to a triangle.
      [1913 Webster]

   3. (Geol.) Contemporaneous in origin; as, the equivalent
      strata of different countries.
      [1913 Webster]

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            He owned that, if the Test Act were repealed, the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]

The Collaborative International Dictionary of English v.0.48:

Equivalent \E*quiv"a*lent\, v. t.
   To make the equivalent to; to equal; equivalence. [R.]
   [1913 Webster]

WordNet (r) 3.0 (2006):

equivalent
    adj 1: 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:
           equivalent, tantamount(p)]
    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, equivalent weight,
       combining weight, eq]

Moby Thesaurus II by Grady Ward, 1.0:

252 Moby Thesaurus words for "equivalent":
   Doppelganger, accordant, actual thing, affirmative, agent, agnate,
   agreeable, agreeing, akin, alike, all one, all the same, ally,
   alter ego, alternate, alternative, analogon, analogous, analogue,
   analogy, answerable, answering, associate, at one, backup, balance,
   ballast, brother, carbon copy, change, changeable, changeling,
   close, close copy, close match, coequal, coexistent, coexisting,
   coextensive, cognate, coherent, coincident, coincidental,
   coinciding, commensurate, commutable, commutative, companion,
   comparable, comparison, compatible, compeer, complement,
   complemental, complementary, concordant, concurring, conformable,
   congenator, congener, congenial, congruent, congruous,
   consentaneous, consentient, consideration, consistent, consonant,
   convertible, cooperating, cooperative, coordinate, copy, correlate,
   correlative, correspondent, corresponding, coterminous,
   counterbalance, counterfeit, counterpart, counterpoise,
   counterweight, dead ringer, deputy, ditto, double, dummy,
   duplicate, en rapport, equal, equalizing, equiparant, equipoise,
   equipollent, ersatz, even, exact counterpart, exchange, exchanged,
   facsimile, fake, fellow, fill-in, ghost, ghostwriter,
   give-and-take, harmonious, homograph, homologous, homonym,
   homophone, idem, identic, identical, identical same, image,
   imitation, in accord, in agreement, in rapport, in sync,
   in synchronization, inaccordance, indistinguishable, inharmony,
   interchangeable, interchanged, kindred spirit, like, like-minded,
   likeness, locum tenens, makeshift, makeweight, match, matching,
   mate, metaphor, metonymy, mock, much the same, mutual,
   near duplicate, next best thing, no other, none other, obverse,
   of a kind, of a piece, of a size, of like mind, of one mind,
   offset, on all fours, opposite number, parallel, paralleling, peer,
   pendant, permutable, personnel, phony, picture, pinch,
   pinch hitter, positive, proportionate, provisional, proxy,
   quid pro quo, reciprocal, reciprocating, reciprocative,
   reconcilable, relief, replacement, replica, representative,
   reserve, reserves, retaliatory, returnable, ringer, rival,
   second self, second string, secondary, self-consistent, selfsame,
   setoff, sign, similar, similitude, simulacrum, sister,
   something of value, soul mate, spare, spares, spit and image,
   spitting image, stand-in, standard, stopgap, sub, substituent,
   substitute, substitution, succedaneum, such, suchlike, superseder,
   supplanter, surrogate, swapped, switched, symbiotic, symbol,
   synchronized, synchronous, synecdoche, synonym, synonymous, tally,
   tantamount, temporary, tentative, the like of, the likes of,
   the same, the same difference, third string, tit for tat, token,
   traded, transposed, twin, unanimous, understudy, undifferenced,
   uniform, unisonant, unisonous, utility, utility player, very image,
   very same, vicar, vicarious, vice-president, vice-regent



Bouvier's Law Dictionary, Revised 6th Ed (1856):

EQUIVALENT. Of the same value. Sometimes a condition must be literally
accomplished in forma specifica; but some may be fulfilled by an equivalent,
per oequi polens, when such appears to be the intention of the parties; as,
I promise to pay you one hundred dollars, and then die, my executor may
fulfill my engagement; for it is equivalent to you whether the money be paid
to you b me or by him. Roll. Ab. 451; 1 Bouv. Inst. n. 760.