A polygon is said to be concave if two or more lines that contain a side of the polygon also contain points in the interior of the polygon. A concave polygon is presented in the illustration below with the lines that include sides of the polygon as well as points in the interior.

A polygon is concave if one or more of the internal angles is greater than $180^\circ.$ Only polygons with four or more sides can be concave. See also convex polygon.