(Lat. convexus, arched), a term applied to the exterior surface of any gibbous or rounded body, as a bowl; in opposition to the hollow inner surface of such body, which is said to be concave.

Convexity and concavity are of particular importance in catoptries and dioptries, as applied to lenses and mirrors. A lens when convex on both sides is said to be convexo-convex—otherwise termed a double-convex lens; when plane on one side and convex on the other, plano-convex; and when convex on one side and concave on the other, con- vexo-concave, or concavo-convex if the convex surface be a portion of a smaller sphere than the concave side. See OPTICS.