The Collaborative International Dictionary of English v.0.48:
Convexity \Con*vex"i*ty\, n.; pl. Convexities. [L. convexitas: cf. F. convexit['e].] The state of being convex; the exterior surface of a convex body; roundness. [1913 Webster] A smooth, uniform convexity and rotundity of a globe. --Bentley. [1913 Webster]