The word concave means curving in or hollowed inward. The term's most commonly used to refer to: Concave lens, a lens with inward-curving (concave) surfaces. Concave polygon, a polygon which isn't convex. Concave function, a type of function which's related to convex functions. Concave mirror Concave set In addition, the term concave upwards's used for convex functions, and concave downwards for concave functions.… (More on Concave)