Longitude describes the location of a place east or west of a north-south line called the prime meridian. Longitude is an angular measurement ranging from 0° at the prime meridian to plus or minus 180°. Zero degrees is defined as a point which passes through Greenwich in London, England.
This measurement is important to navigation; the discovery of how to measure it accurately was one of the more important discoveries of the 1700s. See Dava Sobel's book " : The True Story of a Lone Genius Who Solved the Greatest Scientific Problem of His Time" for a good historical overview. This genius was John Harrison who eventually received the Longitude Prize.