wheel bit n. A privilege bit that allows the possessor to perform some
   restricted operation on a timesharing 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 (esp. at university sites). See
   also {root}.


Source: The Jargon File