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.
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   2. Pertaining to, adapted for, or connected with, both land
      and water.
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            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.
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            Not in free and common socage, but in this
            amphibious subordinate class of villein socage.
                                                  --Blackstone.
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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]