Greenwich gives its name to the prime meridian adopted internationally in 1884.
The meridian passes through the former Royal Observatory at Greenwich, in southeast London, and is defined as 0 degrees longitude. At the International Meridian Conference in Washington, D.C., in 1884, delegates selected the Greenwich meridian as the international reference for longitude and timekeeping.
Greenwich was already widely used by navigators because British nautical charts and the Admiralty’s almanacs had made the meridian influential. The conference also established a global system of time zones based on the prime meridian, although local adoption developed gradually.
Modern geodesy defines the reference meridian more precisely through the International Terrestrial Reference Frame. The line marked at the observatory is therefore not exactly the modern GPS zero-longitude line; the difference is roughly 100 metres. “Greenwich Mean Time” also has historical importance, but Coordinated Universal Time is the modern international standard.