[an.tag.o.nist] n (1594) 1: one that contends with or opposes another: adversary, opponent
2: an agent of physiological antagonism: as a: a muscle that contracts with and limits the action of an agonist with which it is paired--called also antagonistic muscle b: a chemical that acts within the body to reduce the physiological activity of another chemical substance (as an opiate); esp: one that opposes the action on the nervous system of a drug or a substance occurring naturally in the body by combining with and blocking its nervous receptor--compare agonist 2b