agonist agonist /ˈæɡənɪst/

noun

1. (biochemistry) a drug that can combine with a receptor on a cell to produce a physiological reaction  ( agonist )
   (biochemistry) a drug that can combine with a receptor on a cell to produce a physiological reaction
   
2. A muscle that contracts while another relaxes  ( agonist )
   a muscle that contracts while another relaxes
    When bending the elbow the biceps are the agonist
   when bending the elbow the biceps are the agonist
   
3. Someone involved in a contest or battle (as in an agon)  ( agonist )
   someone involved in a contest or battle (as in an agon)
    The two agonists faced each other, ready to prove their strength.
   The two agonists faced each other, ready to prove their strength.
   
   
4. The principal character in a work of fiction  ( agonist , protagonist )
   the principal character in a work of fiction