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)