Definition - What does Longitude mean?
Longitude is the measurement of distance to the west or east of the prime meridian. It can be measured with the help of imagery vertical lines which run from the North to the South Pole. These vertical lines are called meridians. The degrees of the Longitudes are divided into 60 minutes and each minute is further divided into seconds. Its largest degree of the Longitude is about 69 miles (111 kilometers) at its widest. Longitudes are related to the latitudes which are the measurement of distance of south or north of the Equator.
Petropedia explains Longitude
Longitude is also measured in minutes, seconds and degrees in the arc amount that is created by drawing a line first from the center of the Earth to the intersection of the Equator and prime meridian and then drawing another line from the center of the Earth to a point away on the Equator. It is measured to be 180 degrees both west and east of the prime meridian. The exact position is determined with the help of a grid or framework established by combining the parallels of the latitude and meridians of the longitude in reference to the Equator and prime meridian.