agonist

Concise Oxford English Dictionary © 2008 Oxford University Press:
agonist /ˈagənɪst/

▶noun

• 1 Biochemistry a substance which initiates a physiological response when combined with a receptor.

• 2 Anatomy a muscle whose contraction moves a part of the body directly.

• 3 a protagonist.

– derivatives
agonism noun.

– origin early 20th cent.: from Gk agōnistēs ‘contestant’, from agōn ‘contest’; cf. antagonist.