/ˈagənist/
Noun, pl. agonists
1. An agonist is a chemical that binds to some receptor of a cell and triggers a response by that cell. Agonists often mimic the action of a naturally occurring substance. (wikipedia.org)
2. (Pharmacology)
A molecule that combines with a receptor on a cell to trigger physiological reaction. An example is an acetylcholine being the agonist that combines with the cholinergic receptor. (biology-online.org)
3. (Histology)
A muscle that contracts while another muscle relaxes, e.g. when bending the elbow the biceps are the agonist. (biology-online.org)