The Collaborative International Dictionary of English v.0.48:

Contraposition \Con"tra*po*si"tion\, n. [Pref. contra- +
   position: cf. f. conterposition.]
   1. A placing over against; opposite position. [Obs.] --F.
      Potter.
      [1913 Webster]

   2. (Logic) A so-called immediate inference which consists in
      denying the original subject of the contradictory
      predicate; e. g.: Every S is P; therefore, no Not-P is S.
      [1913 Webster]

Moby Thesaurus II by Grady Ward, 1.0:

70 Moby Thesaurus words for "contraposition":
   antagonism, antipathy, antithesis, backlash, bucking, challenge,
   clashing, collision, con, conflict, confrontation, confutation,
   contention, contradiction, contradistinction, contraindication,
   contrariety, contrast, contravention, contraversion, counteraction,
   counterposition, counterworking, crankiness, cross-purposes,
   crosscurrent, crossing, crotchetiness, denial, disagreement,
   discrepancy, dissent, friction, head wind, hostility, impugnation,
   impugnment, inconsistency, inimicalness, interference, kick,
   negation, nonconformity, opposing, oppositeness, opposition,
   opposure, oppugnance, oppugnancy, oppugnation, perverseness,
   perversity, polarity, reaction, rebutment, rebuttal, recalcitrance,
   recoil, refusal, rejection, renitency, repercussion, repugnance,
   resistance, revolt, showdown, standing against, swimming upstream,
   traversal, undercurrent