The Collaborative International Dictionary of English v.0.48:

Zebu \Ze"bu\, n. [?. z['e]bu; of uncertain origin.] (Zool.)
   A bovine mammal (Ros Indicus) extensively domesticated in
   India, China, the East Indies, and East Africa. It usually
   has short horns, large pendulous ears, slender legs, a large
   dewlap, and a large, prominent hump over the shoulders; but
   these characters vary in different domestic breeds, which
   range in size from that of the common ox to that of a large
   mastiff.
   [1913 Webster]

   Note: Some of the varieties are used as beasts of burden, and
         some fore for riding, while others are raised for their
         milk and flesh. The Brahmin bull, regarded as sacred by
         the Hindoos, also belongs to this species. The male is
         called also Indian bull, Indian ox, Madras ox,
         and sacred bull.
         [1913 Webster]

WordNet (r) 3.0 (2006):

zebu
    n 1: domesticated ox having a humped back and long horns and a
         large dewlap; used chiefly as a draft animal in India and
         east Asia