Nick Name

Common usage:

A nickname is a shortened or familiar form of a person's proper name.

Internet chat and Usenet usage:

The term nickname refers to an alias or handle used to identify someone online, and may have little relation to their proper name.

Usage on this Wiki:

On this wiki, RealNames are preferred and the norm. A common-usage nickname may qualify as a "real name", but an alias does not.

(See RealNamesPleaseDiscussion if you want to argue about it.)

InterWiki usage:

Also used in inter-wiki as prefix to a wiki site - 'c2:RecentChanges' with the 'c2:' being the nickname for 'http://c2.com/cgi/wiki?'.