Definition of plane in discrete mathematics: A plane can be drawn on a plane, and any two sides have no other intersection except the endpoint. Such a plan is called a plan.

Note: Some drawings have intersections on the surface, but if you change the drawing method, there will be no intersections. Such a diagram is also a plan.

Definition of non-planar graph: No matter how the geometric shape of a graph changes, the edges of the graph always cross except at the nodes. Such a graph is non-planar.