Pseudonym

Full Title or Meme

A Pseudonym is a name used in place of a real name.

Many web sites require users to have a persistent user name which is unique on their site to identify the user.
As a general rule Web Sites, and as a practical rule Identifier or Attribute Providers require that the pseudonym be unique within their domain; thus the pseudonym@domain.tld will be a valid URI.
In contexts where names are not validated (of low Assurance) the problem arises that trolls many adopt the name of some well-known person to be able to make statements that falsely appear to be from the real person.^[1]

User may wish to use some name that is not associated with their real name when the web site requires a persistent user name.

Users are often asked to use their email address as a local user name since the email address is known to be a URI and hence unique in the internet.
When the user has an option, they often pick some sort of description name, such as a gamertag in a role playing game, which is tested for uniqueness within the relevant name domain.
When a user needs to provide some sort of validated attribute for the duration of a session at a web site, a session ID might provide sufficient security to bind the verified attribute for the session duration.

Synonyms for a Pseudonym include user name, display name, gamertag, non de guerre, Subject ID (sid) or (on Facebook) Fake Name subject to arbitrary termination.
Anonym is not used in the context of identity as it does not provide one. It may be used as the condition (Anonymous) of a user prior to accepting (1) a cookie, (2) an HTTPS connection or (3) a request for an Identifier.
↑ Jack Nicas, Oprah, Is That You? Most Likely, It's Not. 2018-07-08 New York Times page BU1