A pseudonym (/ˈsjuːdənɪm/; from Ancient Greek ψευδώνυμος (pseudṓnumos) 'falsely named') or alias (/ˈeɪli.əs/) is a fictitious name that a person assumes for a particular purpose, which differs from their original or true name (orthonym). This also differs from a new name that entirely or legally replaces an individual's own. Many pseudonym holders use them because they wish to remain anonymous and maintain privacy, though this may be difficult to achieve as a result of legal issues.

Pseudonyms include stage names, user names, ring names, pen names, aliases, superhero or villain identities and code names, gamertags, and regnal names of emperors, popes, and other monarchs. In some cases, it may also include nicknames. Historically, they have sometimes taken the form of anagrams, Graecisms, and Latinisations.

Pseudonyms should not be confused with new names that replace old ones and become the individual's full-time name. Pseudonyms are "part-time" names, used only in certain contexts: to provide a more clear-cut separation between one's private and professional lives, to showcase or enhance a particular persona, or to hide an individual's real identity, as with writers' pen names, graffiti artists' tags, resistance fighters' or terrorists' noms de guerre, computer hackers' handles, and other online identities for services such as social media, online gaming, and internet forums. Actors, musicians, and other performers sometimes use stage names for a degree of privacy, to better market themselves, and other reasons.

In some cases, pseudonyms are adopted because they are part of a cultural or organisational tradition; for example, devotional names are used by members of some religious institutes, and "cadre names" are used by Communist party leaders such as Trotsky and Lenin.

A collective name or collective pseudonym is one shared by two or more persons, for example, the co-authors of a work, such as Carolyn Keene, Erin Hunter, Ellery Queen, Nicolas Bourbaki, or James S. A. Corey.

Publius was a pseudonym collectively used by James Madison, Alexander Hamilton and John Jay in writing The Federalist Papers. Because these were a series of individually written papers, use of the collective pseudonym has led future researchers to debate the particular identity of authors of specific papers.

The term pseudonym is derived from the Greek word "ψευδώνυμον" (pseudṓnymon), literally "false name", from ψεῦδος (pseûdos) 'lie, falsehood' and ὄνομα (ónoma) "name". The term alias is a Latin adverb meaning "at another time, elsewhere".

Sometimes people change their names in such a manner that the new name becomes permanent and is used by all who know the person. This is not an alias or pseudonym, but in fact a new name. In many countries, including common law countries, a name change can be ratified by a court and become a person's new legal name.