A polygon with notches or concave areas along its perimeter (as opposed to convex polygon). A five-pointed star is an example of a concave polygon.

To determine if a polygon is concave, apply the rubber band test. If an imaginary rubber band placed around the outside of a polygon would not touch all sides, then the polygon is concave. Concave polygons must be triangulated (formed into various triangles) before they can be rendered.