1. (

• protagonist ,
• agonist ,

) the principal character in a work of fiction ;

2. (

• agonist

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

3. (

• agonist

) a muscle that contracts while another relaxes; "when bending the elbow the biceps are the agonist" ;

4. (

• agonist

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