adversary

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. An opponent; an enemy.
2. n. The Devil; Satan. Often used with the.

Century Dictionary and Cyclopedia

1. Opposed; opposite to; adverse: antagonistic: as, “adversary forces,” Bp. King.
2. In law, having an opposing party, in contradistinction to unopposed: as, an adversary suit.
3. n. One who acts adversely or inimically; an unfriendly opponent or antagonist; an enemy.
4. n. Specifically [capitalized] The devil; Satan as the general enemy of mankind: as, the wiles of the Adversary.
5. n. An opponent in a contest; one who contends against another or strives for victory; a contestant.
6. n. 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.
7. To antagonize; oppose.

Wiktionary

1. n. An opponent or rival.

GNU Webster's 1913

1. n. 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.
2. adj. Archaic Opposed; opposite; adverse; antagonistic.
3. adj. (Law) Having an opposing party; not unopposed.

WordNet 3.0

1. n. someone who offers opposition