The Collaborative International Dictionary of English v.0.48:

Aves \A"ves\, n. pl. [L., pl. of avis bird.] (Zool.)
   The class of Vertebrata that includes the birds.
   [1913 Webster]

   Note: Aves, or birds, have a complete double circulation,
         oviparous reproduction, front limbs peculiarly modified
         as wings; and they bear feathers. All existing birds
         have a horny beak, without teeth; but some Mesozoic
         fossil birds (Odontornithes) had conical teeth inserted
         in both jaws. The principal groups are: Carinat[ae],
         including all existing flying birds; Ratit[ae],
         including the ostrich and allies, the apteryx, and the
         extinct moas; Odontornithes, or fossil birds with
         teeth.
         [1913 Webster]

   Note: The ordinary birds are classified largely by the
         structure of the beak and feet, which are in direct
         relation to their habits. See Beak, Bird,
         Odontonithes.
         [1913 Webster]

WordNet (r) 3.0 (2006):

Aves
    n 1: (ornithology) the class of birds [syn: Aves, class
         Aves]