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 K5, a complete graph with 5 vertices, is not planar
any graph that contains a subgraph of Ka,b where both a>2 and b>2 is not planar