chromatic number <mathematics> 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://utc.edu/~cpmawata/petersen/lesson8.htm). Eric Weisstein's World Of Mathematics (http://mathworld.wolfram.com/ChromaticNumber.html). The Geometry Center (http://geom.umn.edu/~zarembe/grapht1.html). (2000-03-18)