The strong chromatic number, S(G), of an n-vertex graph G is the smallest number k such that after adding kn/k-n isolated vertices to G and considering any partition of the vertices of the resulting graph into disjoint subsets V1, . . . , Vn/k of size k each, one can find a proper k-vertex-coloring of the graph such that each part Vi, i = 1, . . . , n/k, contains exactly one vertex of each color.
Penny E. Haxell