Source : Webster's Revised Unabridged Dictionary (1913)

Adversary \Ad"ver*sa*ry\, a.
   1. Opposed; opposite; adverse; antagonistic. [Archaic] --Bp.
      King.

   2. (Law) Having an opposing party; not unopposed; as, an
      adversary suit.

Adversary \Ad`ver*sa*ry\, n.; pl. {Adversaries}. [OE.
   adversarie, direct fr. the Latin, and adversaire, fr. OF.
   adversier, aversier, fr. L. adversarius (a.) turned toward,
   (n.) an adversary. See {Adverse}.]
   One who is turned against another or others with a design to
   oppose

Source : WordNet®

adversary
     n : someone who offers opposition [syn: {antagonist}, {opponent},
          {opposer}, {resister}] [ant: {agonist}]