The Collaborative International Dictionary of English v.0.48:

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 or resist them; a member of an opposing or hostile
   party; an opponent; an antagonist; an enemy; a foe.
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         His ancient knot of dangerous adversaries. --Shak.
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         Agree with thine adversary quickly.      --Matt. v. 25.
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         It may be thought that to vindicate the permanency of
         truth is to dispute without an adversary. --Beattie.
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   The Adversary, The Satan, or the Devil.
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   Syn: Adversary, Enemy, Opponent, Antagonist.

   Usage: Enemy is the only one of these words which necessarily
          implies a state of personal hostility. Men may be
          adversaries, antagonists, or opponents to each other
          in certain respects, and yet have no feelings of
          general animosity. An adversary may be simply one who
          is placed for a time in a hostile position, as in a
          lawsuit, an argument, in chess playing, or at fence.
          An opponent is one who is ranged against another
          (perhaps passively) on the opposing side; as a
          political opponent, an opponent in debate. An
          antagonist is one who struggles against another with
          active effort, either in a literal fight or in verbal
          debate.
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