A villain (also known as a "black hat" or "bad guy"; the feminine form is villainess) is a stock character, whether based on a historical narrative or one of literary fiction. Random House Unabridged Dictionary defines such a character as "a cruelly malicious person who is involved in or devoted to wickedness or crime; scoundrel; or a character in a play, novel, or the like, who constitutes an important evil agency in the plot". The antonym of a villain is a hero.

The villain's structural purpose is to serve as the opposition of the hero character and their motives or evil actions drive a plot along. In contrast to the hero, who is defined by feats of ingenuity and bravery and the pursuit of justice and the greater good, a villain is often defined by their acts of selfishness, evilness, arrogance, cruelty, and cunning, displaying immoral behavior that can oppose or pervert justice.