a person who is opposed to, struggles against, or competes with another; opponent; adversary.

the adversary of the hero or protagonist of a drama or other literary work

Iago is the antagonist of Othello.

Physiology . a muscle that acts in opposition to another. Compare agonist ( def. 3 ) .

Dentistry . a tooth in one jaw that articulates during mastication or occlusion with a tooth in the opposing jaw.

Pharmacology . a drug that counteracts the effects of another drug.

noun

ally , protagonist , supporter

noun

adversary , angries , bad person , bandit * , competitor , contender , crip , enemy , foe , match , opponent , opposer , opposite number , oppugnant , [[rivalnotes:protagonist is greek for \'first (important) actor\' or \'first struggler\' and by extension is used for a person who drives the action in a situation or is the main character in a literary work; the protagonist is the hero or heroine and an antagonist is his/her principal opponent]] , opposition , oppositionist , resister , assailant , attacker , battler , foeman , rival