An agonist is a substance that binds to a receptor and triggers a response in the cell. An agonist is the opposite of an antagonist in the sense that while an antagonist also binds to the receptor, the antagonist fails to activate the receptor and actually blocks it from activation by agonists. A partial agonist (such as buspirone, aripiprazole, bifeprunox or norclozapine) activates a receptor but does not cause as much of a physiological change as does a full agonist. A co-agonist works with other co-agonists to produce the desired effect together. The receptors of the human body work by being stimulated or inhibited by natural (such as hormones and neurotransmitters) or synthetic (such as drugs) agonists and antagonists. To see how an agonist may activate a receptor see this link. Recently a novel theory called Functional Selectivity has been proposed that broadens the conventional definition of pharmacology.
Stems from the Greek agonistes, 'contestant', from agon, 'contest'. An agonist is a chemical contestant or contender.