What is the name of the line at 0 degrees longitude?

The line at 0 degrees longitude is the prime meridian.

The modern internationally recognized prime meridian passes through the Royal Observatory at Greenwich in London. It serves as the reference line from which longitude is measured eastward and westward to 180 degrees. The meridian itself is an imaginary line rather than a visible feature on Earth’s surface.

The Greenwich meridian became the global standard after the 1884 International Meridian Conference in Washington, D.C. At that time, most maritime charts and ships already used Greenwich-based longitude, helping delegates choose it over competing reference meridians.

The astronomical instrument used at Greenwich does not define today’s geodetic zero line perfectly. Modern satellite-based coordinate systems place the internationally accepted reference meridian roughly 102 metres east of the historic Airy Transit Circle. “Prime meridian” is longitude’s reference, while the Equator is latitude’s zero line.