Source : Webster's Revised Unabridged Dictionary (1913)

Antithesis \An*tith"e*sis\, n.; pl. {Antitheses}. [L., fr. Gr.
   ?, fr. ? to set against, to oppose; ? against + ? to set. See
   {Thesis}.]
   1. (Rhet.) An opposition or contrast of words or sentiments
      occurring in the same sentence; as, ``The prodigal robs
      his heir; the miser robs himself.'' ``He had covertly shot
      at Cromwell; he how openly aimed at the Queen.''

   2. The second of two clauses forming an antithesis.

   3. Opposition; contrast.

Source : WordNet®

antithesis
     n 1: exact opposite; "his theory is the antithesis of mine"
     2: the juxtaposition of contrasting words or ideas to give a
        feeling of balance
     [also: {antitheses} (pl)]