Ag"o*nist (&?;), n. [Gr. &?;.] One who contends for the prize in public games. [R.]

• someone involved in a contest or battle (as in an agon)
  
• a muscle that contracts while another relaxes
       "when bending the elbow the biceps are the agonist"
  
• (Biochemistry) a drug that can combine with a receptor on a cell to produce a physiological reaction