If a simple polygon is not convex, it is called concave. At least one internal angle of a concave polygon is larger than 180 degrees. A concave polygon is often called re-entrant polygon (but in some cases the latter term has a different meaning). ...

See also:

Convex polygon, Convex polygon - Concave polygons

Read more here: » Convex polygon: Encyclopedia II - Convex polygon - Concave polygons