Greenwich Mean Time (GMT), the name for mean solar time of the longitude (0°) of the Royal Greenwich Observatory in England. The meridian at this longitude is called the prime meridian or Greenwich meridian.

Greenwich Mean Time was used for clearly designating epoch by avoiding confusing references to local time systems (zones). In accord with astronomical tradition, astronomers had always used Greenwich Mean Astronomical Time (GMAT)—the same as GMT but with the day beginning at noon. In 1925 GMT was adopted by astronomers so that the astronomical day began at midnight, the same time as the civil day. Some confusion in terminology resulted, though, and in 1928 the International Astronomical Union changed the designation of the standard time of the Greenwich meridian to Universal Time, which remains in general use in a modified form as Coordinated Universal Time (UTC). The term GMT is still used for some purposes—e.g., navigation in English-speaking countries. (Source Encyclopaedia Britannica)