0 degrees of longitude.

The meridian at Greenwich, England, from which longitude is measured east and west.

A historical note is that Greenwich was not always the location of the prime meridian. The French had established their own meridian in competition with Greenwich's, but lost the battle for acceptance.

This is relevant now because the French government is (in part) celebrating the millennium by planting a row of trees along the line of their prime meridian.

Quite a few different countries established their own prime meridians. The United States used Washington, DC as the prime meridian for a while in the 19th century.

The International Meridian Conference, held in Washington, DC in 1884, designated the Meridian of Greenwich as the standard prime meridian. France and Brazil abstained from the vote.