agonist in WordNet English dictionary

1. (biochemistry) a drug that can combine with a receptor on a cell to produce a physiological reaction
   agonist 
   
2. a muscle that contracts while another relaxes
   "when bending the elbow the biceps are the agonist"
   agonist 
   
3. someone involved in a contest or battle (as in an agon)
   agonist 
   
4. the principal character in a work of fiction
   protagonist  agonist