Source : Free On-Line Dictionary of Computing

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://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)