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

Moby Thesaurus II by Grady Ward, 1.0:

112 Moby Thesaurus words for "amphibian":
   Reptilia, Surinam toad, aeroboat, air, airy nothing, angiosperm,
   anguine, annual, aquatic plant, batrachian, biennial,
   boat seaplane, bubble, bullfrog, caecilian, clipper, colubriform,
   cosmopolite, crawling, creeping, croaker, crocodilian, cutting,
   deciduous plant, dicot, dicotyledon, eft, ephemeral, ether,
   evergreen, exotic, floatplane, flowering plant, flying boat, frog,
   froggy, fungus, gametophyte, grass frog, gymnosperm, hellbender,
   hoppytoad, hoptoad, hydrophyte, hydroplane, illusion, leopard frog,
   lizardlike, midwife toad, mist, monocot, monocotyl, mud puppy,
   newt, ophidian, paddock, perennial, phantom, pickerel frog, plant,
   polliwog, polycot, polycotyl, polycotyledon, repent, reptant,
   reptile, reptilelike, reptilian, reptiliform, reptiloid,
   salamander, saurian, seaplane, seed plant, seedling, serpentiform,
   serpentile, serpentine, serpentlike, serpentoid, shadow, siren,
   slithering, smoke, snakelike, snaky, spermatophyte, spirit,
   sporophyte, spring frog, tadpole, thallophyte, thin air, toad,
   toadish, tree frog, tree toad, triennial, vapor, vascular plant,
   vegetable, viperiform, viperish, viperlike, viperoid, viperous,
   vipery, water dog, waterplane, weed, wood frog