The Free On-line Dictionary of Computing (30 December 2018):

chromatic number

    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)