An epithet (from Ancient Greek ἐπίθετον (epítheton) 'adjective', from ἐπίθετος (epíthetos) 'additional'), also byname, is a descriptive term (word or phrase) known for accompanying or occurring in place of a name and having entered common usage. It has various shades of meaning when applied to seemingly real or fictitious people, divinities, objects, and binomial nomenclature. It can also be a descriptive title: for example, Pallas Athena, Phoebus Apollo, Alfred the Great, Suleiman the Magnificent, and Władysław I the Elbow-high. Many English monarchs have traditional epithets: some of the best known are Edward the Confessor, William the Conqueror, Richard the Lionheart, Æthelred the Unready, John Lackland and Bloody Mary.

The word epithet can also refer to an abusive, defamatory, or derogatory phrase. This use as a euphemism is criticized by Martin Manser and other proponents of linguistic prescription. H. W. Fowler complained that "epithet is suffering a vulgarization that is giving it an abusive imputation."