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"

Moby Thesaurus II by Grady Ward, 1.0:

81 Moby Thesaurus words for "avatar":
   Buddha, Christophany, Jagannath, Juggernaut, Kurma, Matsya,
   Narsinh, Parshuram, Rama, Satanophany, Vaman, Varah, angelophany,
   apparition, appearance, appearing, arising, catabolism, catalysis,
   coming, coming into being, coming-forth, consubstantiation,
   disclosure, displacement, dissemination, embodiment, emergence,
   epiphany, evidence, evincement, exposure, expression, forthcoming,
   heterotopia, incarnation, indication, issuance, manifestation,
   materialization, materializing, metabolism, metagenesis,
   metamorphism, metamorphosis, metastasis, metathesis,
   metempsychosis, mutant, mutated form, mutation, occurrence,
   opening, permutation, pneumatophany, presentation, proof,
   publication, realization, reincarnation, revelation, rise, rising,
   showing, showing forth, sport, theophany, transanimation,
   transfiguration, transfigurement, transformation, transformism,
   translation, translocation, transmigration, transmogrification,
   transmutation, transposition, transubstantiation, unfolding,
   unfoldment



V.E.R.A. -- Virtual Entity of Relevant Acronyms (February 2016):

AVATAR
       Advanced Video Attribute Terminal Assembler and Recreator (BBS)


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.


The Free On-line Dictionary of Computing (30 December 2018):

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)