What is a poikilotherm?

Poikilotherm organisms are those whose internal body temperature varies and so can maintain their body temperature according to the environment.

You can consider cold blooded animals as poikilotherm but some poikilotherm can maintain their body temperature constant because of the constant environment temperature. eg. frogs and reptiles.

Ever tried keeping frog in a water and slowly increasing the temperature of the water. The frog can maintain its body temperature according to the water but upto some point only. (obviously it will also die in boiling hot water :D)