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

admin
 /ad?min'/, n.

    Short for ?administrator?; very commonly used in speech or on-line to refer
    to the systems person in charge on a computer. Common constructions on this
    include sysadmin and site admin (emphasizing the administrator's role as a
    site contact for email and news) or newsadmin (focusing specifically on
    news). Compare postmaster, sysop, system mangler.


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

system administration
admin
sysadmin
system administrator

    Activities performed by a system administrator (or
   "admin", "sysadmin", "site admin") such as monitoring security
   configuration, managing allocation of user names and
   passwords, monitoring disk space and other resource use,
   performing backups, and setting up new hardware and
   software.

   system administrators often also help users, though in a large
   organisation this may be a separate job.

   Compare postmaster, sysop, system management, systems
   programmer.

   [Other tasks?]

   (1999-05-02)