An archenemy, archfoe, archvillain or nemesis (sometimes spelled arch-enemy, arch-foe or arch-villain originated around the mid-16th century) is the principal enemy of someone or something. In fiction, it is a character who is the hero/protagonist's worst enemy.

The word archenemy or arch-enemy originated around the mid-16th century, from the words arch- (from Greek "arkhos" meaning "most important") and enemy.