Prime Meridian (or initial meridian)

By convention determined meridian on the Earth’s surface from which longitude is measured.

Widely spread was the in 1634 established Prime Meridian through the Canarian island Ferro which was declared as 20 degrees to the west of Paris in 1720. (17 degrees 40 minutes in the west of Greenwich.)

In 1884 (international conference in Washington D.C.) the meridian through the old observatory of Greenwich was established as the Prime Meridian.