A clique of an undirected graph is a set C $$of vertices such that for any two vertices u$,$v $$in C$,$there is an edge between u $$and v$.$For example, in the below graph, $\{1,2,4,6\}$is a clique of size $4,$and there are no cliques of larger size. Thus $\{1,2,4,6\}$is a clique of maximum size.