Angular distance, in degrees, minutes, and seconds of a point east or west of the Greenwich meridian. A meridian is a great circle on the surface of the Earth passing through the geographical poles and any given point on the Earth's surface. All points on a given meridian have the same longitude.
Definition adapted from: glossary of cartography and map terminology, http://www.worldatlas.com/mapterms.htm, sensu Maps for America, 3rd edition.
For more, visit the Wikipedia entry for longitude, URL: http://en.wikipedia.org/wiki/Longitude