1. a drug or other chemical agent that inhibits the action of another substance. For example, an antagonist may combine with the substance to alter and thus inactivate it (chemical antagonism); an antagonist may reduce the effects of the substance by binding to the same receptor without stimulating it, which decreases the number of available receptors (pharmacological antagonism); or an antagonist may bind to a different receptor and produce a physiological effect opposite to that of the substance (physiological antagonism).

2. a contracting muscle whose action generates force opposing the intended direction of movement. This force may serve to slow the movement rapidly as it approaches the target or it may help to define the movement end point. Compare agonist. —antagonism n. —antagonistic adj.