In Hindu mythology, an avatar is the incarnation of a god.
1. Among people working on virtual reality and cyberspace interfaces, an avatar is an icon or representation of a user in a shared virtual reality. The term is sometimes used on MUDs.
2. [CMU, Tektronix] root, superuser. There are quite a few Unix machines on which the name of the superuser account is `avatar' rather than `root'. This quirk was originated by a CMU hacker who found the terms `root' and `superuser' unimaginative, and thought `avatar' might better impress people with the responsibility they were accepting.
Source: the public domain Jargon file