n. 1. An agent, for example a drug or hormone, that works to reduce the action of an agonist - for example, by preventing an agonist agent from binding with a binding site. It may bind in a competitive manner or to a completely different binding site on the receptor (see pharmacological antagonism). 2. Also refers to the opposing actions of muscles that move by utilizing the antagonist-agonist action to go in the intended direction or slow down when approaching the target (see also equilibrium-point model). 3. The thwarter of another individual, using adversary action.