Poikilotherm

A poikilotherm is an organism whose body temperature varies considerably under normal circumstances. The opposite is a homeotherm - an organism that maintains a constant body temperature.

For example, as opposed to mammals, who maintain a near constant body temperature, the body temperature of a lizard can change considerably between when its basking in the sun and when it is under water.

Poikilothermy is one aspect of what is meant by cold-blooded, which usually includes ectothermy.