The Collaborative International Dictionary of English v.0.48:

Emu \E"mu\, n. [Cf. Pg. ema ostrich, F. ['e]mou, ['e]meu, emu.]
   (Zo["o]l.)
   A large Australian bird, of two species (Dromaius
   Nov[ae]-Hollandi[ae] and D. irroratus), related to the
   cassowary and the ostrich. The emu runs swiftly, but is
   unable to fly. [Written also emeu and emew.]
   [1913 Webster]

   Note: The name is sometimes erroneously applied, by the
         Brazilians, to the rhea, or South American ostrich.
         [1913 Webster]

   Emu wren. See in the Vocabulary.
      [1913 Webster]

WordNet (r) 3.0 (2006):

Dromaius novaehollandiae
    n 1: large Australian flightless bird similar to the ostrich but
         smaller [syn: emu, Dromaius novaehollandiae, Emu
         novaehollandiae]