The wikipedia definition of "planar graph" says:
In graph theory, a planar graph is a graph that can be embedded in the plane, i.e., it can be drawn on the plane in such a way that its edges intersect only at their endpoints.
I think this is a quite good description, and one quickly understands what is meant by "planar graph". But to me, the definition seems to be informal. How can one formally define the concepts "drawing on the plane" and "the edges don't cross"?
