- 1 Biochemistry a substance that initiates a physiological response when combined with a receptor. Compare with antagonist.
- 2 Anatomy a muscle whose contraction moves a part of the body directly. Often contrasted with antagonist.
- 3another term for protagonist.
early 20th century: from Greek agōnistēs 'contestant' (a sense reflected in English in the early 17th century), from agōn 'contest'