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).
