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]

Moby Thesaurus II by Grady Ward, 1.0:

97 Moby Thesaurus words for "antagonist":
   Roscius, actor, actress, adversary, anti, antihero, archenemy,
   assailant, bad guy, barnstormer, bit, bit part, bitter enemy, cast,
   character, character actor, character man, character woman,
   child actor, combatant, competition, competitor, con, contender,
   cue, devil, diseur, diseuse, dramatizer, enemy, fat part, feeder,
   foe, foeman, foil, heavy, hero, heroine, histrio, histrion,
   ingenue, juvenile, lead, lead role, leading lady, leading man,
   leading woman, lines, match, matinee idol, mime, mimer, mimic,
   monologist, mummer, open enemy, opponent, opposer, opposing party,
   opposite camp, opposition, oppugnant, pantomime, pantomimist, part,
   person, personage, piece, playactor, player, protagonist,
   protean actor, public enemy, reciter, role, side, soubrette,
   stage performer, stage player, stooge, straight man, straight part,
   stroller, strolling player, supporting character, supporting role,
   sworn enemy, the loyal opposition, the opposition, theatrical,
   thespian, title role, trouper, utility man, villain, walk-on,
   walking part