Agonist refers to a drug that effectively mimics the action of a natural chemical messenger within the body.
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, it fails to activate the receptor and actually blocks it from activation by agonists. A partial agonist activates a receptor but does not cause as much of a physiological change as does a full agonist. 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.
For example, in treating Parkinson disease, the long-used drug levodopa can cause uncontrollable, jerky body movements called dyskinesias that can inhibit a person's ability to function. Dopamine agonists mimic the effects of dopamine in the brain by stimulating dopamine receptors with a lower risk of the uncontrollable and irreversible dyskinesias often associated with levodopa therapy. The word "agonist" comes from the Late Latin agnista, contender, from the Greek agnists, contestant, from agn, contest. An agonist is a chemical contestant or contender.
Agonist refers to a muscle or muscle group that is described as being primarily responsible for a specific joint movement when contracting.
Other definition:
Agonist is a term used in neuroscience that refers to a chemical substance that effectively increases the activity of a Neurotransmitter by imitating its effects.