Ectotherm

An ectotherm is an organism whose body temperature is controlled by external conditions. These organisms are ectothermic. Ectotherms are commonly described as cold-blooded, although the term is misleading as some ectothermic bacteria that live in hot springs have body temperatures well over 45 °C. While all ectotherms generate heat through their metabolic processes, that heat is quickly dispersed and the organism's body temperature remains not much higher than if its metabolism were shut down.

The opposite of ectothermy is endothermy - organisms that generate heat internally for the purpose of staying warm.

Many ectotherms are also poikilotherms, meaning that their body temperature changes considerably under normal circumstances. Ectotherms that are not poikilothermic include some tropical fish that live in water that stays a relatively constant temperature.

Word origin
In Greek, ecto means outside and therm means heat or hot. Thus, an ectotherm gets its heat from the outside environment.