The Collaborative International Dictionary of English v.0.48:

Contra \Con"tra\
   A Latin adverb and preposition, signifying against, contrary,
   in opposition, etc., entering as a prefix into the
   composition of many English words. Cf. Counter, adv. &
   pref.
   [1913 Webster]

WordNet (r) 3.0 (2006):

Contra
    n 1: a member of the guerrilla force that opposed a left-wing
         government in Nicaragua