The Collaborative International Dictionary of English v.0.48:

Amphibious \Am*phib"i*ous\, a. [Gr. 'amfi`bios living a double
   life, i. e., both on land in water; 'amfi` + bi`os life.]
   1. Having the ability to live both on land and in water, as
      frogs, crocodiles, beavers, and some plants.
      [1913 Webster]

   2. Pertaining to, adapted for, or connected with, both land
      and water.
      [1913 Webster]

            The amphibious character of the Greeks was already
            determined: they were to be lords of land and sea.
                                                  --Hare.
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   3. Of a mixed nature; partaking of two natures.
      [1913 Webster]

            Not in free and common socage, but in this
            amphibious subordinate class of villein socage.
                                                  --Blackstone.
      [1913 Webster]

WordNet (r) 3.0 (2006):

amphibious
    adj 1: relating to or characteristic of animals of the class
           Amphibia [syn: amphibious, amphibian]
    2: operating or living on land and in water; "amphibious
       vehicles"; "amphibious operations"; "amphibious troops";
       "frogs are amphibious animals" [ant: aquatic,
       terrestrial]

Moby Thesaurus II by Grady Ward, 1.0:

46 Moby Thesaurus words for "amphibious":
   adaptable, adjustable, all-around, amalgamated, ambidextrous,
   ambiguous, ambivalent, blended, combined, complex, composite,
   compound, compounded, conglomerate, dappled, eclectic, equivocal,
   fifty-fifty, flexible, generally capable, half-and-half,
   heterogeneous, indiscriminate, intricate, ironic, jumbled,
   many-sided, medley, mingled, miscellaneous, mixed, motley,
   multifaceted, multinational, multiracial, patchy, pluralistic,
   promiscuous, resourceful, scrambled, supple, syncretic,
   thrown together, two-handed, varied, versatile