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]

WordNet (r) 3.0 (2006):

convexity
    n 1: the property possessed by a convex shape [syn: convexity,
         convexness]
    2: a shape that curves or bulges outward [syn: convex shape,
       convexity]