antagonist

Etymology

From Latin antagonista, from Ancient Greek ἀνταγωνιστής (antagōnistḗs, “opponent”) (ἀντί (antí, “against”) + ἀγωνιστής (agōnistḗs, “a combatant, pleader, actor”)), from ἀνταγωνίζεσθαι (antagōnízesthai, “antagonize”).

noun

1. An opponent or enemy.
2. One who antagonizes or stirs.
3. (biochemistry) A chemical that binds to a receptor but does not produce a physiological response, blocking the action of agonist chemicals.
   2001: The calcium antagonists represent one of the top ten classes of prescription drugs in terms of commercial value, with worldwide sales of nearly $10 billion in 1999. — Leslie Iversen, Drugs: A Very Short Introduction (Oxford 2001, p. 41)
4. (authorship) The main character or force opposing the protagonist in a literary work or drama.
5. (anatomy) A muscle that acts in opposition to another.
   A flexor, which bends a part, is the antagonist of an extensor, which extends it.