The Collaborative International Dictionary of English v.0.48:

Amphibian \Am*phib"i*an\ (-an), a. (Zool.)
   Of or pertaining to the Amphibia; as, amphibian reptiles.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Amphibian \Am*phib"i*an\, n. (Zool.)
   One of the Amphibia.
   [1913 Webster]

WordNet (r) 3.0 (2006):

amphibian
    adj 1: relating to or characteristic of animals of the class
           Amphibia [syn: amphibious, amphibian]
    n 1: a flat-bottomed motor vehicle that can travel on land or
         water [syn: amphibian, amphibious vehicle]
    2: an airplane designed to take off and land on water [syn:
       amphibian, amphibious aircraft]
    3: cold-blooded vertebrate typically living on land but breeding
       in water; aquatic larvae undergo metamorphosis into adult
       form