Plain English Meaning
How far east or west a place is on the Earth, measured from a starting line that passes through Greenwich, England. The lines you see running top-to-bottom on a globe are lines of longitude.
Full Definition
An angular measurement, expressed in degrees east or west of the Prime Meridian (which runs through Greenwich, England), that specifies a position's east-west location on the Earth's surface. Lines of longitude, called meridians, run from the North Pole to the South Pole and are used together with latitude to identify any point on the globe.
Context Anchor
Pilots see longitude in chart coordinates, GPS positions, airport data, and when a text compares position on a graph to position on a map.