thumb|class=skin-invert-image|200px|An example of a concave polygon.
A simple polygon that is not convex is called concave, non-convex or reentrant. A concave polygon will always have at least one reflex interior angle—that is, an angle with a measure that is between 180° degrees and 360° degrees exclusive.
Polygon
Some lines containing interior points of a concave polygon intersect its boundary at more than two points.
According to Euclidean geometry, a triangle can never be concave, but there exist concave polygons with n sides for any n > 3. An example of a concave quadrilateral is the dart.
At least one interior angle does not contain all other vertices in its edges and interior.
The convex hull of the concave polygon's vertices, and that of its edges, contains points that are exterior to the polygon.