The fraction or ratio characterized by the map scale which describes how one map unit represents the number of actual units on the earth’s surface.
Definition adapted from: Krygier, John and Denis Wood. 2005. Making maps: a visual guide to map design for GIS, accessed through http://makingmaps.owu.edu
For more, visit the Wikipedia entry for map scale, URL: http://en.wikipedia.org/wiki/Scale_(map)