CONVEX

From Agepedia

CONVEX (from the Latin convexus, vaulted, arched); rising in a circular form; the contraiy to concave. Thus the inside of a watchglass is concave, the outer surface convex. The mathematician defines a curved line convex on the side on which the point of intersection of two tangents falls, and concave on the opposite side. Convexity and concavity are of particular importance in catoptrics and dioptrics, as applied to mirrors and lenses.