WordNet (r) 3.0 (2006):

Greenwich Mean Time
    n 1: the local time at the 0 meridian passing through Greenwich,
         England; it is the same everywhere [syn: Greenwich Mean
         Time, Greenwich Time, GMT, universal time, UT,
         UT1]

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

Greenwich Mean Time

    (GMT) The local time on the Greenwich
   meridian, based on the hypothetical mean sun (which averages
   out the effects of the Earth's elliptical orbit and its tilted
   axis).  GMT is the basis of the civil time for the UK.

   In 1925 the reference point was changed from noon to midnight
   and it was recommended that the term "Universal Time" should
   be used for the new GMT.

   Authorities disagreed on whether GMT equates with UT0 or
   UT1, however the differences between the two are of the
   order of thousandths of a second.  GMT is no longer used for
   scientific purposes.

   (2001-08-02)