A pseudonym is a fictitious alternative to a person's legal name (see alias). In some cases, pseudonyms are adopted because it is part of a cultural or organizational tradition, as in the case of devotional names used by members of some religious orders and "cadre names" used by Communist party leaders such as Trotsky and Stalin.

Pseudonyms are also used to hide an individual's identity, as with writers' pen names, graffiti artists, resistance fighters or terrorists' noms de guerre and computer hackers' handles. An example is of the well known fictional spy character James Bond concealing his identity by using the pseudonym James St. John Smith in the film A View To A Kill. Actors, musicians, and other performers sometimes use a stage name to mask their original ethnic background, particularly in the early to mid-1900s. Stage names are also used to create a name which better matches their stage persona, as in the case of hip hop artists such as Ol' Dirty Bastard (who was known under at least six aliases); Black metal performers such as Nocturno Culto; and hardcore punk singers such as "Rat" of Discharge.

A collective name or collective pseudonym is one shared by two or more persons. This is sometimes used by the co-authors of a work, such as Ellery Queen.

The term is derived from , pseudónymon – literally "given a name by error, lie name" from , pseúdos – the lie and , ónoma – the name); pseudo + -onym: false name. A pseudonym is distinct from an allonym, which is the name of another actual person, assumed by someone in authorship of a work of art; such as when ghostwriting a book or play, or in parody, or when using a "front" name such as by screenwriters blacklisted in Hollywood in the 1950s and 1960s.

Monarchies

In many monarchies, the sovereign is allowed to choose a regnal name by which he or she will be known. This official name may differ from his or her first name and may not even be one of his or her given names at birth.

A sovereign may choose not to use his or her first name for many reasons. Some, such as George VI of the United Kingdom (born Albert Frederick Arthur George), may wish to make a connection between their reign and that of a previous sovereign (in his case, his father, George V). Others, such as Queen Victoria (born Alexandrina Victoria of Kent), may never have been known by their original first name. Other sovereigns might select a regnal name to emphasize the legitimacy of the succession or even to indicate a change in policy or religion.

In Japan, the Emperor's personal name is never used as a regnal name: he is referred to by the name of his regnal era, and after his death his name is officially changed to that of the era. It is a severe breach of etiquette in Japan to refer to the current Emperor's personal name either in speech or in writing unless absolutely required by law. This does not apply to those outside Japan, however, which explains why Japanese and non-Japanese use different names for the Emperor. For instance, Emperor Hirohito was known within Japan as Emperor Showa.