1. In physiology, a muscle whose contraction is directly responsible for the movement of a part of the body: In his biology class, Jeff learned that the agonist in bending the elbow was the biceps!
2. A contender for prizes in competitions; a protagonist: Jack was an agonist who took part in the sports contest at school.

An agonist can be an individual who has a leading role in a drama presentation, or is the principal character in a story.