The Collaborative International Dictionary of English v.0.48:

Antithesis \An*tith"e*sis\, n.; pl. Antitheses. [L., fr. Gr.
   ?, fr. ? to set against, to oppose; 'anti` 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."
      [1913 Webster]

   2. The second of two clauses forming an antithesis.
      [1913 Webster]

   3. Opposition; contrast.
      [1913 Webster]

WordNet (r) 3.0 (2006):

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