The geographic longitude is one of the two co-ordinates of a point onto the Earth sphere and that the angle u between its meridian plane and the plane of a conventional determined Prime Meridian.
Starting from this Prime Meridian the geographic longitudes are counted eastern and western from 0 degrees up to 180 degrees or (geodesy) positive to the East from 0 degrees up to 360 degrees.
Points with the same longitude have the same local time. Thus you also can interpret differences in longitude as time: 15 degrees are equivalent to one hour.