Source : Webster's Revised Unabridged Dictionary (1913)

Antagonist \An*tag"o*nist\, a.
   Antagonistic; opposing; counteracting; as, antagonist schools
   of philosophy.

Antagonist \An*tag"o*nist\, n. [L. antagonista, Gr. ?; ? against
   + ? combatant, champion, fr. ?: cf. F. antagoniste. See
   {Antagonism}.]
   1. One who contends with another, especially in combat; an
      adversary; an opponent.

            Antagonist of Heaven's Almigthy King. --Milton.

            Our antagonists in these controversies. --Hooker.

   2. (Anat.) A muscle which acts in opposition to another; as a
      flexor, which bends a part, is the antagonist of an
      extensor, which extends it.

   3. (Med.) A medicine which opposes the action of another
      medicine or of a poison when absorbed into the blood or
      tissues.

   Syn: Adversary; enemy; opponent; toe; competitor. See
        {Adversary}.

Source : WordNet®

antagonist
     n 1: someone who offers opposition [syn: {adversary}, {opponent},
           {opposer}, {resister}] [ant: {agonist}]
     2: a muscle that relaxes while another contracts; "when bending
        the elbow the triceps are the antagonist"
     3: a drug that neutralizes or counteracts the effects of
        another drug [ant: {synergist}]