WordWeb Online

1. The principal character in a work of fiction
   - protagonist
    
2. Someone involved in a contest or battle (as in an agon)
    
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

Derived forms: agonists

Type of: antagonistic muscle, character, contestant, drug, fictional character, fictitious character

Antonym: adversary

Encyclopedia: Agonist