Agonist

Word: [noun] agonist ((biochemistry) a drug that can combine with a receptor on a cell to produce a physiological reaction)

Word: [noun] agonist (a muscle that contracts while another relaxes)

Example: When bending the elbow the biceps are the agonist

Word: [noun] agonist (someone involved in a contest or battle (as in an agon))

Word: [noun] agonist (the principal character in a work of fiction)

Synonyms: protagonist