Metamorphosis is a biological process by which an animal physically develops after birth or hatching, involving a conspicuous and relatively abrupt change in the animal's form or structure through cell growth and differentiation. Some insects, amphibians, molluscs, crustaceans, Cnidarians,echinoderms and tunicates undergo metamorphosis, which is usually (but not always) accompanied by a change of habitat or behaviour.

Scientific usage of the term is exclusive, and is not applied to general aspects of cell growth, including rapid growth spurts. References to "metamorphosis" in mammals are imprecise and only colloquial.