agonist

noun

1. 1

   (biochemistry) a drug that can combine with a receptor on a cell to produce a physiological reaction

2. 2

   a muscle that contracts while another relaxes

   when bending the elbow the biceps are the agonist

3. 3

   someone involved in a contest or battle (as in an agon)

4. 4

   the principal character in a work of fiction