The smallest number of colours necessary to colour the nodes of a graph so that no two adjacent nodes have the same colour. See also: four colour map theorem. {Graph Theory Lessons (http://www.utc.edu/~cpmawata/petersen/lesson8.htm)}. {Eric Weisstein' s World Of Mathematics (http://mathworld.wolfram.com/ChromaticNumber.html)}. {The Geometry Center (http://www.geom.umn.edu/~zarembe/grapht1.html)}. (2000-03-18)
