The Collaborative International Dictionary of English v.0.48:

Ovolo \O"vo*lo\, n. [It. ovolo, uovolo, fr. L. ovum an egg. Cf.
   Ovule.] (Arch.)
   A round, convex molding. See Illust. of Column.
   [1913 Webster]

   Note: In Roman work it is usually a quarter circle in
         section; in Greek work it is flatter, and is equivalent
         to the echinus; that is, it has in section the elastic
         curve of the shell of the sea urchin. In mediaeval
         architecture it is not distinguishable from the
         multitude of convex moldings, of all sections, which
         are used.
         [1913 Webster]

WordNet (r) 3.0 (2006):

ovolo
    n 1: a convex molding having a cross section in the form of a
         quarter of a circle or of an ellipse [syn: ovolo,
         thumb, quarter round]