an·tag·o·nist [an tágg?nist] (plural an·tag·o·nists) noun 1. opponent: somebody or something opposing or in conflict with another a former supporter turned antagonist 2. character in conflict with hero: a major character in a book, play, or movie whose values or behavior are in conflict with those of the protagonist or hero 3. pharmacology neutralizing agent: a drug that neutralizes the effect of a substance on the body 4. physiology opposing muscle: a muscle that acts with and limits the action of another muscle