Concave and convex polygons
A polygon is convex if and only if the line segment connecting any two points in the polygonal region lies entirely within the region. If a polygon is not convex, it is called concave or nonconvex.

Convex polygons	Concave polygons

A concave polygon (caves in) has at least one diagonal outside the figure. Also, at least one interior angle is greater than 180º.