An endotherm is an organism whose body temperature is controlled by generating body heat through shivering or increased metabolism. These organisms are endothermic. The opposite of endothermy is ectothermy - organisms whose temperature is controlled by external conditions.
Endotherms are commonly called warm-blooded, although the term is misleading as some ectothermic bacteria that live in hot springs have body temperatures well over 45 °C, yet do not fit the definition of an endotherm.
In Greek, endo means inside and therm means heat or hot. Thus, an endotherm gets its heat from inside itself.