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

superuser
 n.

    [Unix] Syn. root, avatar. This usage has spread to non-Unix
    environments; the superuser is any account with all wheel bits on. A more
    specific term than wheel.


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

superuser

   [Unix] Synonym root, avatar.  This usage has spread to
   non-Unix environments; the superuser is any account with all
   wheel bits on.  A more specific term than wheel.

   [Jargon File]