WordNet (r) 3.0 (2006):

showdown
    n 1: a hostile disagreement face-to-face [syn: confrontation,
         encounter, showdown, face-off]

Moby Thesaurus II by Grady Ward, 1.0:

30 Moby Thesaurus words for "showdown":
   antagonism, antipathy, antithesis, clashing, climax, collision,
   conflict, confrontation, contradiction, contradistinction,
   contraindication, contraposition, contrariety, contrast,
   counterposition, cross-purposes, disagreement, discrepancy,
   hostility, inconsistency, inimicalness, moment of truth,
   oppositeness, opposition, opposure, oppugnance, oppugnancy,
   perversity, polarity, repugnance