Universal Transverse Mercator
The Universal Transverse Mercator (UTM) projection system is a grid-based method of specifying locations on the surface of the Earth. It is used to identify locations on the earth, but differs from the traditional method of latitude and longitude in several respects. The UTM system is not a map projection, but rather employs a series of zones based on specifically defined Transverse Mercator projections.
See also Wikipedia: http://en.wikipedia.org/wiki/Universal_Transverse_Mercator