antagonist

noun

• 1a person who actively opposes or is hostile to someone or something; an adversary: the woman was forcing her antagonist’s face into the mud

• 2 Biochemistry a substance which interferes with or inhibits the physiological action of another: LSD is a serotonin antagonistCompare with agonist.

• 3 Anatomy a muscle whose action counteracts that of another specified muscle. Compare with agonist.