ag·o·nist 
(ăgə-nĭst)
(ăgə-nĭst)n.
1. Physiology A contracting muscle that is resisted or counteracted by another muscle, the antagonist.
2. A substance that can combine with a receptor on a cell to initiate signal transduction.
3. One involved in a struggle or competition.
[Late Latin agōnista, contender, from Greek agōnistēs, from agōn, contest; see AGONY.]