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

Moby Thesaurus II by Grady Ward, 1.0:

81 Moby Thesaurus words for "contra":
   a rebours, adverse to, against, against the grain,
   against the tide, against the wind, antipode, antipodes, antipole,
   antithesis, antonym, arsy-varsy, at cross-purposes,
   at cross-purposes with, at daggers, at daggers drawn, at issue,
   at odds, at variance, at war with, athwart, by contraries, con,
   contradictory, contrarily, contrariously, contrariwise, contrary,
   contrawise, converse, conversely, counter, counter to,
   counterbalance, countercheck, counterpoint, counterpoise,
   counterpole, counterterm, cross, dead against, eyeball-to-eyeball,
   facing, foil, fronting, in conflict with, in confrontation,
   in flat opposition, in hostile array, in opposition,
   in opposition to, inverse, inversely, just the opposite,
   nay rather, obverse, offset, on the contrary, opposed to, opposite,
   opposite number, oppositely, otherwise, over, per contra,
   quite the contrary, rather, reverse, setoff, the contrary,
   the other side, to the contrary, topsy-turvy, tout au contraire,
   toward, up in arms, upside down, versus, vice versa, vis-a-vis,
   with crossed bayonets



Bouvier's Law Dictionary, Revised 6th Ed (1856):

CONTRA. Over; against; opposite to anything: as, such a case lays down a
certain principle; such other case, contra.