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

Moby Thesaurus II by Grady Ward, 1.0:

62 Moby Thesaurus words for "antithesis":
   antagonism, anteposition, antipathy, antipode, antipodes, antipole,
   antonym, clashing, collision, con, conflict, confrontation,
   confrontment, contention, contra, contradiction, contradictory,
   contradistinction, contraindication, contraposition, contrariety,
   contrary, contrast, converse, counter, counterbalance,
   countercheck, counterpoint, counterpoise, counterpole,
   counterposition, counterterm, cross-purposes, disagreement,
   discrepancy, foil, hostility, inconsistency, inimicalness, inverse,
   obverse, offset, opposing, opposite, opposite number, oppositeness,
   opposition, opposure, oppugnance, oppugnancy, perversity,
   polar opposition, polarity, polarization, posing against,
   repugnance, reverse, setoff, showdown, the contrary,
   the other side, vis-a-vis