Before revision: The crossing number cr(G) of a graph G is the minimum number of crossings over all possible drawings of G in the plane. Analogously, the k-planar crossing number of G, denoted by cr_{k edges of G in k disjoint planes.
After revision: The crossing number cr(G) of a graph G is the minimum number of crossings over all possible drawings of the edges of G in k disjoint planes.