Which meridian was selected as the international prime meridian in 1884?

The Greenwich Meridian was selected as the international prime meridian in 1884.

Delegates at the International Meridian Conference in Washington, D.C., voted in October 1884 to use the meridian passing through the Royal Observatory at Greenwich, England, as longitude 0 degrees. Twenty-five nations were represented, and the proposal passed with 22 votes in favour, one against, and two abstentions.

Greenwich had already become widely used in navigation and mapmaking because many nautical charts and tables used Greenwich Mean Time. The decision did not make every national map instantly identical, and France continued using the Paris Meridian for some time. Today, the internationally defined zero-degree reference is based on the International Reference Meridian, which lies about 102 metres east of the historic Greenwich line because modern geodesy uses Earth-centred measurements.