A concave polygon is a polygon with not all interior angles smaller in measure than a straight angle (180°).

A concave polygon is a polygon with al least one interior angle larger in measure than a straight angle.

In a concave polygon, not all diagonals will occur in the interior of the polygon, as illustrated the the example below left.

In a convex polygon, all diagonals will occur in the interior of the polygon, as illustrated the the example above right.