Former name for mean solar time of the longitude (0k) of the former Royal Observatory at Greenwich, England, or Greenwich meridian.
GMT was used to avoid potentially confusing references to local time systems (zones). In accord with tradition, 0000 GMT (denoting the start of a solar day) occurred at noon. In 1925 the numbering system for GMT was changed so that the day (like the civil day) began at midnight. Some confusion resulted, and in 1928 the International Astronomical Union changed the designation of the standard time of the zero meridian to Universal Time , which remains in general use. The term GMT is still used for some purposes (including navigation) in English-speaking countries.