imaginary line used to indicate 0° longitude that passes through Greenwich, a borough of London, and terminates at the North and South poles. An international conference held in Washington, D.C., in 1884 designated “the meridian passing through the centre of the transit instrument at the Observatory of Greenwich as the initial meridian for longitude.” The observatory (renamed the Royal Greenwich Observatory) was moved to Hailsham, East Sussex, during the 1950s, but the original site continues to serve as the location for 0° longitude.

As the prime meridian, the north-south line at Greenwich is used as the reference for all other meridians of longitude, which are numbered east or west of it. The Greenwich meridian also serves as the basis for the world's standard time zone system. The mean solar time at Greenwich is now called Universal Time (q.v.) and was formerly called Greenwich Mean Time (q.v.). Theoretically, standard time becomes successively one hour earlier at each 15° longitude west of the Greenwich meridian and one hour later at each 15° longitude east. (See standard time.)