Watch Terminology Dictionary

This refers to the mean solar time measured at the Greenwich Observatory. It is abbreviated as GMT (Greenwich Mean Time). The "mean" is used because it is calculated mean solar time, not true solar time. Originally the standard time used within the United Kingdom, by 1884, the North American railway network and most of the world's shipping industry were using Greenwich Mean Time. As a result, at the International Meridian Conference in Washington in the same year, it was designated as the prime meridian for world standard time, and Greenwich Mean Time remained the world's standard time until 1965.