This refers to a class of vertebrate animals that spend part of their time on land and part in the water; they are an intermediate form between fishes and reptiles. Amphibians must return to the water to breed and they have distinct larval and adult forms. Members of this class include frogs, toads, and salamanders.
This word is derived from the Greek word amphibios, which means "living a double life".
From the BioTech Dictionary at http://biotech.icmb.utexas.edu/. For further information see the BioTech homenode.