opponent

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. One that opposes another or others in a battle, contest, controversy, or debate. See Synonyms at enemy.
2. adj. Acting against an antagonist or opposing force: opponent armies.
3. adj. Located in front.

Century Dictionary and Cyclopedia

1. Situated in front; opposite; standing in the way.
2. Opposing; antagonistic; adverse.
3. In anatomy, bringing together or into opposition; having the action of an opponens. See opponens.
4. n. One who opposes; an adversary; an antagonist; one who supports the opposite side in controversy, disputation, or argument, or in a contest of any kind.
5. n. One who takes part in an opponency; the person who begins a dispute by raising objections to a tenet or doctrine: correlative to defendant or respondent. Synonyms Adversary, Antagonist, Opponent, etc. (see adversary), rival, competitor, opposer.

Wiktionary

1. n. An individual or group who is a rival of another.
2. n. One who attempts to stop the progression of.
3. n. One who objects to.
4. n. In team sports, a player of an opposite team.

GNU Webster's 1913

1. adj. Situated in front; opposite; hence, opposing; adverse; antagonistic.
2. n. One who opposes; an adversary; an antagonist; a foe.
3. n. One who opposes in a disputation, argument, or other verbal controversy; specifically, one who attacks some thesis or proposition, in distinction from the respondent, or defendant, who maintains it.

WordNet 3.0

1. n. a contestant that you are matched against
2. adj. characterized by active hostility
3. n. someone who offers opposition