Source : Free On-Line Dictionary of Computing

su
     
         (substitute user) The
        {Unix} command which allows you to become another user after
        entering their {password}.  su is most often used without
        arguments in which case it defaults to user {root}.  Some
        versions of Unix only allows this command to be used by
        members of the {wheel} group.
     
        {Unix manual page}: su(1).
     
        (1996-09-08)