Source : WordNet®

time zone
     n : any of the 24 regions of the globe (loosely divided by
         longitude) throughout which the same standard time is
         used

Source : Free On-Line Dictionary of Computing

time zone
     
        One of approximately 24 longitudinal divisions of the globe,
        nominally 15 degrees wide, in which clocks show the same time.
        Some zones follow the boundaries of states or territories,
        others differ from neighbouring zones by more or less than one
        hour.
     
        Computers can be programmed to take into account the time zone
        each user is working in, which is not necessarily the same as
        the zone the computer is in.
     
        See also {TZ}.
     
        (1997-07-20)