The Collaborative International Dictionary of English v.0.48:

Avatar \Av`a*tar"\ ([a^]v`[.a]*t[add]r"), n. [Skr. avat[^a]ra
   descent; ava from + root t[.r] to cross, pass over.]
   1. (Hindu Myth.) The descent of a deity to earth, and his
      incarnation as a man or an animal; -- chiefly associated
      with the incarnations of Vishnu.
      [1913 Webster]

   2. An incarnation, embodiment or personification of a
      principle, quality, or attitude; -- used of people, mostly
      in a positive sense as a manifestation of a behavior or
      character worthy of admiration.
      [1913 Webster +PJC]

            Martha Stewart, the home-and-hearth avatar whose
            products are now available at Kmart stores, is
            making upscale design touches like 200-thread-count
            cotton bed sheets something that most every American
            can aspire to.                        --Leslie
                                                  Kaufman (N. Y.
                                                  Times, May 7,
                                                  1999).
      [1913 Webster]

WordNet (r) 3.0 (2006):

avatar
    n 1: a new personification of a familiar idea; "the embodiment
         of hope"; "the incarnation of evil"; "the very avatar of
         cunning" [syn: embodiment, incarnation, avatar]
    2: the manifestation of a Hindu deity (especially Vishnu) in
       human or superhuman or animal form; "the Buddha is considered
       an avatar of the god Vishnu"

The Free On-line Dictionary of Computing (19 January 2023):

avatar
av

   1.  An image representing a user in a
   multi-user virtual reality (or VR-like, in the case of
   Palace) space.

   2. (CMU, Tektronix) root, superuser.  There are quite a
   few Unix computers on which the name of the superuser
   account is "avatar" rather than "root".  This quirk was
   originated by a CMU hacker who disliked the term
   "superuser", and was propagated through an ex-CMU hacker at
   Tektronix.

   [Jargon File]

   (1997-09-14)


The Jargon File (version 4.4.7, 29 Dec 2003):

avatar
 n.

    [in Hindu mythology, 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.