A meridian (or line of longitude) is an imaginary arc on the Earth's surface from the North Pole to the South Pole that connects all locations running along it with a given longitude. The position of a point on the meridian is given by the latitude. Each meridian is perpendicular to all circles of latitude at the intersection points. Each is also the same size, being half of a great circle on the Earth's surface and therefore measuring 20,003.93 km.

Since the meridian that passes through Greenwich, England, establishes the meaning of zero degrees of longitude, or the Prime Meridian, any other meridian is identified by the angle, referenced to the center of the earth as vertex, between where it and the prime meridian cross the equator. As there are 360 degrees in a circle, the meridian on the opposite side of the earth from Greenwich (which forms the other half of a circle with the one through Greenwich) is 180° longitude, and the others lie between 0° and 180° of West longitude in the Western Hemisphere (west of Greenwich) and between 0° and 180° of East longitude in the Eastern Hemisphere (east of Greenwich). You can see the lines of longitude on most maps.

The term "meridian" comes from the Latin meridies, meaning "midday"; the sun crosses a given meridian midway between the times of sunrise and sunset on that meridian. The same Latin stem gives rise to the terms A.M. and P.M. used to disambiguate hours of the day when using the 12-hour clock.