Source : Free On-Line Dictionary of Computing

wheel bit
     
        A privilege bit that allows the possessor to perform some
        restricted operation on a {time-sharing} system, such as read
        or write any file on the system regardless of protections,
        change or look at any address in the running monitor, crash or
        reload the system, and kill or create jobs and user accounts.
        The term was invented on the TENEX operating system, and
        carried over to TOPS-20, XEROX-IFS, and others.  The state of
        being in a privileged logon is sometimes called "wheel mode".
        This term entered the Unix culture from TWENEX in the
        mid-1980s and has been gaining popularity there (especially at
        university sites).  See also {root}.