The Collaborative International Dictionary of English v.0.48:

Amphibia \Am*phib"i*a\, n. pl. [See Amphibium.] (Zool.)
   One of the classes of vertebrates.
   [1913 Webster]

   Note: The Amphibia are distinguished by having usually no
         scales, by having eggs and embryos similar to those of
         fishes, and by undergoing a complete metamorphosis, the
         young having gills. There are three living orders: (1)
         The tailless, as the frogs (Anura); (2) The tailed
         (Urodela), as the salamanders, and the siren group
         (Sirenoidea), which retain the gills of the young
         state (hence called Perennibranchiata) through the
         adult state, among which are the siren, proteus, etc.;
         (3) The C[oe]cilians, or serpentlike Amphibia
         (Ophiomorpha or Gymnophiona), with minute scales
         and without limbs. The extinct Labyrinthodonts also
         belonged to this class. The term is sometimes loosely
         applied to both reptiles and amphibians collectively.
         [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Amphibium \Am*phib"i*um\, n.; pl. L. Amphibia; E.
   Amphibiums. [NL., fr. Gr. ? (sc. ? an animal). See
   Amphibious.]
   An amphibian.
   [1913 Webster]

WordNet (r) 3.0 (2006):

amphibia
    n 1: the class of vertebrates that live on land but breed in
         water; frogs; toads; newts; salamanders; caecilians [syn:
         amphibia, class Amphibia]