The Collaborative International Dictionary of English v.0.48:

Cornu \Cor"nu\ (k[^o]r"n[-u]), n.; pl. Cornua (-n[-u]*[.a]).
   [L.]
   A horn, or anything shaped like or resembling a horn.
   [1913 Webster]

WordNet (r) 3.0 (2006):

cornu
    n 1: (anatomy) any structure that resembles a horn in shape