Definition from Wiktionary, the free dictionary
From Ancient Greek ἀγωνιστής (agōnistēs, “combatant, champion”)
agonist (plural agonists)
- Someone involved in a contest or battle (as in an agon).
- The muscle that contracts while the other relaxes.
- "when bending the elbow the biceps are the agonist"
- (biochemistry) A molecule that can combine with a receptor on a cell to produce a physiological reaction.
- "acetylcholine is an agonist at the a cholinergic receptor"
Derived terms 
someone involved in a contest or battle
a muscle that contracts while another relaxes