The Prime Meridian is the 0-degree longitude line that passes through Greenwich in London.
It provides the starting reference for measuring longitude east or west around Earth. In 1884, delegates at the International Meridian Conference in Washington, D.C., selected the meridian passing through the Royal Observatory at Greenwich as the world’s prime meridian for maps and timekeeping. Greenwich Mean Time became an important reference for international standard time.
The modern international reference is technically based on the International Reference Meridian, which lies slightly east of the historic Greenwich line because satellite-based geodesy models Earth more precisely. On the ground, visitors can stand across the historic meridian line at Greenwich. The Prime Meridian is not the Equator: longitude measures position east and west, while latitude measures position north and south. The International Date Line is another convention, roughly opposite it.