opponent

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun One that opposes another or others, as in a battle, contest, or debate.
• noun One who is hostile to an idea or cause.
• adjective Acting against an antagonist or opposing force.

from The Century Dictionary.

• Situated in front; opposite; standing in the way.
• Opposing; antagonistic; adverse.
• In anatomy, bringing together or into opposition; having the action of an opponens. See opponens.
• noun 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.
• noun 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.

from the GNU version of the Collaborative International Dictionary of English.

• adjective Situated in front; opposite; hence, opposing; adverse; antagonistic.
• noun One who opposes; an adversary; an antagonist; a foe.
• noun 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.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun An individual or group who is a rival of another.
• noun One who attempts to stop the progression of.
• noun One who objects to.
• noun In team sports, a player of an opposite team.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a contestant that you are matched against
• adjective characterized by active hostility
• noun someone who offers opposition