Convex means curved outward, or having positive curvature, as in certain mirrors and lenses; the word comes from the Latin convexus for "vaulted." A biconvex lens has two convex surfaces, not necessarily with the same radii of curvature.

A figure, such as a polygon or a polyhedron, is said to be convex if every line segment that joins two interior points remains inside the figure. Similarly, a set is convex if it contains all the line segments connecting any pair of its points. Compare with concave.