A pseudonym is a name that a person or group assumes for a particular purpose, which differs from his or her original or true name (orthonym). Popular pseudonyms include stage names, noms de plume (pen names), aliases, gamer identifications, anagrams, Graecisms, Latinisations, mystifications, nicknames, and assumed names (and orders) of popes and monarchs.

Pseudonyms are often used to hide an individual's real identity, as with writers' pen names, graffiti artists' tags, resistance fighters' or terrorists' noms de guerre, and computer hackers' handles. Actors, musicians, and other performers sometimes use stage names, for example, to mask their ethnic backgrounds. Employers sometimes require employees to use assigned names to help sell products; for example, a company that does business mostly in one country but locates a call center in another country may require its employees to assume names common in the former country in an effort to draw a more positive or less negative reaction from current and/or prospective customers.