Concave



The word concave means  curving in or hollowed inward. The term is most commonly used to refer to:
 * Concave lens, a lens with inward-curving (concave) surfaces.
 * Concave polygon, a polygon which is not convex.
 * Concave function, a type of function which is related to convex functions.
 * Concave mirror

In addition, the term concave upwards is used for convex functions, and concave downwards for concave functions.