antagonist

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. One who opposes and contends against another; an adversary.
2. n. The principal character in opposition to the protagonist or hero of a narrative or drama.
3. n. Physiology A muscle that counteracts the action of another muscle, the agonist.
4. n. Biochemistry A chemical substance that interferes with the physiological action of another, especially by combining with and blocking its nerve receptor.

Century Dictionary and Cyclopedia

1. n. One who contends with another in combat or in argument; an opponent; a competitor; an adversary.
2. n. In anatomy, a muscle which acts in opposition to another: as, a flexor, which bends a part, is the antagonist of an extensor, which extends it. Synonyms Adversary, Antagonist, Opponent, etc. (see adversary), opposer, rival, assailant.
3. Counteracting; opposing; combating: as, antagonist forces; an antagonist muscle. The flexors and extensors of a limit, as also the abductors and adductors, have to each other the relation of antagonist muscles.

Wiktionary

1. n. An opponent or enemy.
2. n. One who antagonizes or stirs.
3. n. biochemistry A chemical that binds to a receptor but does not produce a physiological response, blocking the action of agonist chemicals.
4. n. The main character or force opposing the protagonist in a literary work or drama.

GNU Webster's 1913

1. n. One who contends with another, especially in combat; an adversary; an opponent.
2. n. (Anat.) A muscle which acts in opposition to another.
3. n. (Med.) A medicine which opposes the action of another medicine or of a poison when absorbed into the blood or tissues.
4. adj. Antagonistic; opposing; counteracting.

WordNet 3.0

1. n. a drug that neutralizes or counteracts the effects of another drug
2. n. someone who offers opposition
3. n. a muscle that relaxes while another contracts