Definitions
1.nounAn opponent or enemy.
2.nounOne who antagonizes or stirs.
3.nounA 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.noun(authorship) The main character or force opposing the protagonist in a literary work or drama.
5.nounA muscle that acts in opposition to another."A flexor, which bends a part, is the antagonist of an extensor, which extends it."