An ectotherm, from the Greek εκτός (ektós) "outside" and θερμός (thermós) "hot", is an organism in which internal physiological sources of heat are of relatively small or quite negligible importance in controlling body temperature. Such organisms (for example frogs) rely on environmental heat sources, which permits them to operate at very economical metabolic rates. Some such animals live in environments in which temperatures are practically constant, as is typical of regions of the abyssal ocean. In contrast where temperature varies so widely as to limit the physiological activities of other kinds of ectotherms, many species habitually seek out external sources of heat or shelter from heat; for example, many reptiles regulate their body temperature by basking in the sun, or seeking shade when necessary. In contrast to ectotherms, endotherms rely largely, even predominantly, on heat from internal metabolic processes.

In ectotherms, fluctuating ambient temperatures affect the body temperature. Such variation in body temperature is called poikilothermy, though the concept is not widely satisfactory and the use of the term is declining. In small aquatic creatures such as Rotifera, the poikilothermy is practically absolute, but other creatures have wider physiological options at their disposal, and they can avoid ambient temperature changes, or moderate their effects.