A maximal completesubgraph of a graph is a clique, and the clique number of a graph is the maximal order of a clique in . Simply, is the maximal order of a complete subgraph of . Some authors however define a clique as any complete subgraph of and refer to the other definition as maximum clique.
Adapted with permission of the author from Modern Graph Theory by Béla Bollobás, published by Springer-Verlag New York, Inc., 1998.