n.
   - a person who actively opposes or is hostile towards someone or something.
 - Biochemistry  a substance which interferes with the physiological action of another.
 - Anatomy  a muscle whose action counteracts that of another muscle.
 
 
  Derivative
  
    - antagonistic adj.
  - antagonistically adv.
 
   
  Etymology
  C16: from Fr. antagoniste or late L. antagonista, from Gk antagōnistēs, from antagōnizesthai (see antagonize); cf. agonist.