Meridian

meridian - [geodesy] A great circle on the earth that passes through the poles, often used synonymously with longitude. Meridians run north-south between the poles. By convention, meridians are labeled with positive numbers that ascend as one moves eastward from the prime meridian, and negative numbers as one moves westward from the prime meridian until the east and west hemispheres meet at the 180-degree line. Meridians can also, however, be labeled with all positive or negative numbers, including positive numbers increasing westward from the prime meridian.

[Category=Geospatial ]

Source: esri, 10 June 2012 09:08:14, http://support.esri.com/en/knowledgebase/GISDictionary/term/abbreviation