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]