agonist

n. (Greek agon = contest). In neuropharmacology, a drug which facilitates synaptic transmission at the receptor site and facilitates the receptor's response. A direct agonist drug attaches to the same binding site as does the neurotransmitter. An indirect agonist attaches to another binding site at the receptor and facilitates action of the neurotransmitter binding site. An agonist operates in contrast to an antagonist, which blocks synaptic transmission.
In muscle physiology, an agonist is a contractile skeletal muscle opposed by another muscle, the antagonist. - VS