antagonist

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

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

from The Century Dictionary.

• noun One who contends with another in combat or in argument; an opponent; a competitor; an adversary.
• noun 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.
• Counteracting; opposing; combating: as, antagonist forces; an antagonist muscle.

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

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

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

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

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

• noun a drug that neutralizes or counteracts the effects of another drug
• noun someone who offers opposition
• noun a muscle that relaxes while another contracts