1630s, "having two modes of existence, of doubtful nature," from Greek amphibia, neuter plural of amphibios "living a double life," from amphi- "of both kinds" (see amphi-) + bios "life" (see bio-).
Formerly used by zoologists to describe all sorts of combined natures (including otters and seals), the biological sense "class of animals between fishes and reptiles that live both on land and in water" and the noun derivative both are first recorded 1835. Amphibia was used in this sense from c.1600 and has been a zoological classification since c.1819.
Vertebrate animals, such as frogs, that live part of their life cycle in the water and the other part on land.
Note: Amphibian is also used to describe things such as vehicles that can operate both on land and in the water.
Note: Amphibians were the first land-dwelling animals to evolve.