What makes an amphibian an amphibian?

Skeleton:

Amphibians have an endoskeleton (meaning an internal skeletal system).

Skin:

Amphibian skin does not have scales and allows fluids to pass through (permeable).  Typically, amphibians have moist, slippery skin.  Some amphibians, like toads, have drier, bumpy skin. 

Feet:

Amphibians typically have webbed toes and skin covered feet.  (Amphibians do not have claws.)

Breathing:

Most adult amphibians breathe through lungs and/or through their skin.  Breathing through the skin is called cutaneous respiration.  To breathe through their skin, the skin must stay moist/wet.

Early in life, amphibians have gills for breathing.  Most will lose their gills as they become adults, however some will keep their gills throughout their life spans.

Body Temperature:

Amphibians are ectotherms.  This means that their body temperature varies with the temperature of their surroundings.

Eggs and Young:

Amphibians lay eggs.  Amphibian eggs must be laid in or near water to stay moist.