The Collaborative International Dictionary of English v.0.48:

Antonym \An"to*nym\, n. [Gr. ? a word used in substitution for
   another; ? + ?, ?, a word.]
   A word of opposite meaning; a counterterm; -- used as a
   correlative of synonym. [R.] --C. J. Smith.
   [1913 Webster]

WordNet (r) 3.0 (2006):

antonym
    n 1: a word that expresses a meaning opposed to the meaning of
         another word, in which case the two words are antonyms of
         each other; "to him the antonym of `gay' was `depressed'"
         [syn: antonym, opposite word, opposite] [ant:
         equivalent word, synonym]

Moby Thesaurus II by Grady Ward, 1.0:

47 Moby Thesaurus words for "antonym":
   antipode, antipodes, antipole, antithesis, articulation, contra,
   converse, counter, counterbalance, countercheck, counterpoint,
   counterpoise, counterpole, counterterm, expression, foil,
   free form, homograph, homonym, homophone, inverse, lexeme,
   linguistic form, locution, logos, metonym, minimum free form,
   monosyllable, obverse, offset, opposite, opposite number,
   polysyllable, reverse, setoff, syllable, synonym, term,
   the contrary, the other side, usage, utterance, verbalism, verbum,
   vis-a-vis, vocable, word