adversary

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

• noun An opponent; an enemy.
• noun The Devil; Satan. Often used with the.

from The Century Dictionary.

• To antagonize; oppose.
• Opposed; opposite to; adverse: antagonistic: as, “adversary forces,” Bp. King.
• In law, having an opposing party, in contradistinction to unopposed: as, an adversary suit.
• noun One who acts adversely or inimically; an unfriendly opponent or antagonist; an enemy.
• noun Specifically [capitalized] The devil; Satan as the general enemy of mankind: as, the wiles of the Adversary.
• noun An opponent in a contest; one who contends against another or strives for victory; a contestant.
• noun Synonyms and Adversary, Antagonist, Opponent, Enemy, Foe. These words vary in strength according as they express spirit, action, or relation. A foe has most of the spirit of enmity, or is actively hostile. The word is more used in poetry than in prose. Enemy, as denoting an opponent in war, or a member of an opposing party, does not necessarily imply personal hostility. Opponent, adversary, and antagonist are less severe in their opposition, and need have no animosity. Opponent is often a passive word; antagonist is always active and personal. A man may be our opponent in an argument or a lawsuit, our adversary in a game, as chess, our antagonist in a wrestling- or boxing-match, or other occasion of strenuous exertion: the choice between the three words depends chiefly upon the measure of activity involved. In the Bible, adversary covers the meaning of all five words.

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

• noun One who is turned against another or others with a design to oppose or resist them; a member of an opposing or hostile party; an opponent; an antagonist; an enemy; a foe.
• noun The Satan, or the Devil.
• adjective Archaic Opposed; opposite; adverse; antagonistic.
• adjective (Law) Having an opposing party; not unopposed.

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

• noun An opponent or rival.

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

• noun someone who offers opposition