In geometry, a polygon can be either convex or concave.
A simple polygon is strictly convex if every internal angle is strictly less than 180 degrees. Equivalently, a polygon is strictly convex if every line segment between two nonadjacent vertices of the polygon is strictly interior to the polygon except at its endpoints.
Every nondegenerate triangle is strictly convex.
A polygon that is not convex is called concave. A concave polygon will always have an interior angle with a measure that is strictly greater than 180 degrees.
It is possible to cut a concave polygon into a set of convex polygons.