An endotherm (Greek: endon = "within", thermē = "heat") is an organism that maintains its body at a metabolically favourable temperature, largely by use of heat set free by its internal bodily functions instead of relying almost purely on ambient heat. Such internally generated heat is mainly an incidental product of the animal's routine metabolism, but under conditions of excessive cold or low activity an endotherm might apply special mechanisms adapted specifically to heat production. Examples include special-function muscular exertion such as shivering, and uncoupled oxidative metabolism such as within brown adipose tissue.
In common parlance, endotherms are characterized as "warm-blooded." The opposite of endothermy is ectothermy, although there is no absolute or clear separation between the nature of endotherms and ectotherms in general.
Read more about Endotherm: Advantages and Disadvantages of An Endothermic Metabolism, Facultative Endothermy, Contrast Between Thermodynamic and Biological Terminology