Four Color Theorem

Theorem
Any planar graph $G$ can be assigned a proper vertex $k$-coloring such that $k \le 4$.

Also known as
In British English, this theorem is spelled four colour theorem.

Some sources hyphenate: four-colo(u)r theorem.

Some sources continue to refer to it as the four colo(u)r problem, acknowledging the controversial nature of its supposed proof.

Also see

 * Five Color Theorem