What is the name of the line at 0° longitude that passes through Greenwich, England?

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

A meridian is an imaginary line running from the North Pole to the South Pole. The Prime Meridian serves as the reference line from which east and west longitude are measured, reaching up to 180 degrees in either direction. Together with the Equator, it forms a basic framework for locating places on Earth.

The Greenwich meridian became internationally accepted at the 1884 International Meridian Conference in Washington, D.C. Delegates selected the meridian crossing the main instrument of the Royal Observatory at Greenwich because British nautical charts and navigation practices were already widely used. The modern international reference is based on the International Reference Meridian, which lies slightly east of the historic line marked at Greenwich.

The Prime Meridian is not the same as the International Date Line. The Prime Meridian is a longitude reference in Greenwich, while the date line broadly follows 180° longitude across the Pacific and marks calendar-date changes.