The Collaborative International Dictionary of English v.0.48:

Rotundness \Ro*tund"ness\, n.
   Roundness; rotundity.
   [1913 Webster]

WordNet (r) 3.0 (2006):

rotundness
    n 1: the roundness of a 3-dimensional object [syn: sphericity,
         sphericalness, globosity, globularness, rotundity,
         rotundness]