The Collaborative International Dictionary of English v.0.48:

antagonist \an*tag"o*nist\ ([a^]n*t[a^]g"[-o]*n[i^]st), n. [L.
   antagonista, Gr. 'antagwnisth`s; 'anti` against + 'agwnisth`s
   combatant, champion, fr. 'agwni`zesqai: cf. F. antagoniste.
   See Antagonism.]
   1. One who contends with another, especially in combat; an
      adversary; an opponent.
      [1913 Webster]

            Antagonist of Heaven's Almighty King. --Milton.
      [1913 Webster]

            Our antagonists in these controversies. --Hooker.
      [1913 Webster]

   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. AS
      [1913 Webster]

   3. (Med.) A substance which opposes the actions of another
      substance in the body, especially a drgu that counteracts
      the effects of another drug.
      [1913 Webster + AS]

   Syn: Adversary; enemy; opponent; foe; competitor. See
        Adversary.
        [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Antagonist \An*tag"o*nist\, a.
   Antagonistic; opposing; counteracting; as, antagonist schools
   of philosophy.
   [1913 Webster] Antagonistical

WordNet (r) 3.0 (2006):

antagonist
    n 1: someone who offers opposition [syn: adversary,
         antagonist, 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]