To be concave is to have a surface or boundary that curves or bulges inward, as does the inner surface of a hemisphere. Concavity is the state of being concave. (A tip to assist in remembering: caves and cavities go in, while convexities go out.)An example of an image in which concavity is an issue: Maurits Cornelis Escher (Dutch, 1898-1972), Convex and Concave, 1955, lithograph, 28 x 33.5 cm. See ambiguity and optical illusion.Also see alar groove, convex, ear, gibbous, mortise, and niche.