Agonist - Definition

n 1: the principal character in a work of fiction [syn:
protagonist, agonist]
2: someone involved in a contest or battle (as in an agon) [ant:
adversary, antagonist, opponent, opposer, resister]
3: a muscle that contracts while another relaxes; "when bending
the elbow the biceps are the agonist"
4: (biochemistry) a drug that can combine with a receptor on a
cell to produce a physiological reaction