Definition of Antagonist

Antagonist: In biochemistry, an antagonist acts against and blocks an action. For example, insulin lowers the level of glucose (sugar) in the blood, whereas another hormone called glucagon raises it; therefore, insulin and glucagon are antagonists.

An antagonist is the opposite of an agonist which stimulates an action. Antagonists and agonists are key players in pharmacology and in the chemistry of the human body.