1. a drug or other chemical agent that binds to a particular receptor and produces a physiological effect, typically one similar to that of the body’s own neurotransmitter at that receptor. There are partial agonists, which stimulate the receptor only somewhat to produce the same physiological effect as the natural neurotransmitter but to a lesser degree, and inverse agonists, which act at the receptor to produce a physiological effect opposite to that produced by another agonist at that same receptor.

2. a contracting muscle whose action generates force in the intended direction. Compare antagonist. —agonism n. —agonistic adj.