Source : Webster's Revised Unabridged Dictionary (1913)

Opponent \Op*po"nent\, n.
   1. One who opposes; an adversary; an antagonist; a foe.
      --Macaulay.

   2. One who opposes in a disputation, argument, or other
      verbal controversy; specifically, one who attacks some
      theirs or proposition, in distinction from the respondent,
      or defendant, who maintains it.

            How becomingly does Philopolis exercise his office,
            and seasonably commit the opponent with the
            respondent, like a long-practiced moderator! --Dr.
                                                  H. More.

   Syn: Antagonist; opposer; foe. See {Adversary}.

Opponent \Op*po"nent\, a. [L. opponens, -entis, p. pr. of
   opponere to set or place against, to oppose; ob (see {Ob-}) +
   ponere to place. See {Position}.]
   Situated in front; opposite; hence, opposing; adverse;
   antagonistic. --Pope.

Source : WordNet®

opponent
     adj : characterized by active hostility; "opponent (or opposing)
           armies" [syn: {opposing}]
     n 1: a contestant that you are matched against [syn: {opposition},
           {opposite}]
     2: someone who offers opposition [syn: {adversary}, {antagonist},
         {opposer}, {resister}] [ant: {agonist}]