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 anti-meridian form a great circle that divides a spheroid, like the Earth, into two hemispheres. The prime meridian is also used as the basis for the worlds time zones.

The prime meridian is an arbitrary line, meaning it could be chosen to be anywhere. However, the International Meridian Conference held in 1884 established the Greenwich meridian as the prime meridian of the world. Before this conference, most countries in Europe adapted their own prime meridian, usually through their capital. The Greenwich meridian passes through the Royal Observatory in Greenwich, England.

Lines of longitude run parallel to each other and are used to measure distance in degrees east or west from the prime meridian. The 180th meridian is the line of longitude that is exactly opposite the prime meridian and is 180° longitude. 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) .