opponent

n.

1. A rival

   competitor, claimant, emulator, striver, contender, challenger, candidate, equal, entrant, the opposition, oppositionist, aspirant, bidder; see also contestant.

   Antonyms supporter*, defender*, abettor.

2. An opposing contestant

   antagonist, disputant, contestant, filibusterer, litigant, learned colleague*; see also player 1.

   Antonyms associate*, partner*, colleague. *

3. An enemy

   enemy, foe, adversary, antagonist, combatant, assailant; see also enemy 1, 2.

opponent, an unemotional word, refers to anyone who is opposed to one, as in a fight, game, contest, or debate; antagonist implies more active opposition, esp. in a struggle for control or power; adversary usually suggests actual hostility in the conflict; enemy may imply actual hatred in the opponent and a desire to injure, or it may simply refer to any member of the opposing group, nation, etc. in a conflict, whether or not there is personal animosity or hostility involved; foe, now a somewhat literary synonym for enemy, connotes more active hostility