Wikipedia
L(h, k)-coloring
L(h, k) coloring in graph theory, is a (proper) vertex coloring in which every pair of adjacent vertices has color numbers that differ by at least h, and any pair of vertices at distance 2 have their colors differ by at least k. When h=1 and k=0, it is the usual (proper) vertex coloring.
L(2,1)-coloring
L(2, 1)-coloring is a particular case of L(h, k)-coloring which is in fact a proper coloring. In L(2, 1)-coloring of a graph, G, the vertices of the graph G is colored or labelled in such a way that the adjacent vertices get labels that differ by at least two. Also the vertices that are at a distance of two from each other get labels that differ by at least one.