What is the name of the line at 0 degrees longitude passing through Greenwich, England?

The line at 0 degrees longitude passing through Greenwich, England, is the Prime Meridian.

Longitude measures position east or west of a chosen reference line. In 1884, representatives at the International Meridian Conference in Washington, D.C., selected the meridian through the Royal Observatory at Greenwich as the international prime meridian for geographic and timekeeping purposes. It became the starting point for measuring longitude from 0 degrees to 180 degrees east or west.

Greenwich was already influential because British nautical charts and the Royal Navy widely used the Greenwich meridian. The conference decision did not mean that every historical map had always used Greenwich; earlier cartographers used different reference meridians.

Modern satellite-based geodesy defines the reference meridian slightly east of the historic observatory line because Earth is modeled using a global coordinate system rather than a physical mark alone. Nevertheless, Greenwich remains the familiar cultural and educational reference. The meridian also helped establish Greenwich Mean Time, later connected to the development of Coordinated Universal Time.