The prime meridian is an imaginary line of longitude that is defined to be 0° and is used as the starting point for measuring distance both east and west around the Earth. It is an arbitrarily chosen meridian in a geographic coordinate system. The prime meridian and its antimeridian (the line of longitude exactly opposite the prime meridian) form a great circle that divides the Earth into the Eastern Hemisphere and the Western Hemisphere. The prime meridian is used as the basis for the worlds time zones. Longitude is a system of imaginary north-south lines called meridians that connect the North Pole to the South Pole, and the prime meridian is the starting point for this measuring system. Lines of longitude east of the prime meridian are numbered from 1 to 179 east (E), while lines of longitude west of the prime meridian are numbered from 1 to 179 west (W).

The prime meridian is also called the Greenwich meridian because it passes through Greenwich, England. Before 1884, mapmakers usually began numbering the lines of longitude on their maps at whichever meridian passed through the site of their national observatory. However, in 1884, scientists decided that the starting point of longitude for everyone would be the meridian running through Britains royal observatory in Greenwich. The International Reference Meridian (IRM) is the only meridian that may now be described as the prime meridian of the world, as it defines 0° longitude by international agreement. The IRM is not a fixed location, but will continue to move as the Earths surface shifts.