in graph theory, a planar graph is a graph where it is possible to make a two dimensional re-representation of the graph where there are no edges crossing

i couldn’t find a mathematical definition for this, but logically speaking, since we are limited to two dimensions, the restrictions are as follows:

any graph that contains a subgraph of , a complete graph with 5 vertices, is not planar

any graph that contains a subgraph of where both and is not planar