An ectotherm (from Ancient Greek ἐκτός (ektós) 'outside' and θερμός (thermós) 'heat'), more commonly referred to as a "cold-blooded animal", is an animal in which internal physiological sources of heat, such as blood, are of relatively small or of quite negligible importance in controlling body temperature. Such organisms (frogs, for example) rely on environmental heat sources, which permit them to operate at very economical metabolic rates.

Some of these animals live in environments where temperatures are practically constant, as is typical of regions of the abyssal ocean and hence can be regarded as homeothermic ectotherms. In contrast, in places 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 addition to a host of other behavioral thermoregulation mechanisms.

In contrast to ectotherms, endotherms rely largely, even predominantly, on heat from internal metabolic processes, and mesotherms use an intermediate strategy.

Because there are more than two categories of temperature control utilized by animals, the terms warm-blooded and cold-blooded have been deprecated as scientific terms.

Various patterns of behavior enable certain ectotherms to regulate body temperature to a useful extent. To warm up, reptiles and many insects find sunny places and adopt positions that maximise their exposure; at harmfully high temperatures they seek shade or cooler water. In cold weather, honey bees huddle together to retain heat. Butterflies and moths may orient their wings to maximize exposure to solar radiation in order to build up heat before take-off. Gregarious caterpillars, such as the forest tent caterpillar and fall webworm, benefit from basking in large groups for thermoregulation. Many flying insects, such as honey bees and bumble bees, also raise their internal temperatures endothermally prior to flight, by vibrating their flight muscles without violent movement of the wings. Such endothermal activity is an example of the difficulty of consistent application of terms such as poikilothermy and homeothermy.

In addition to behavioral adaptations, physiological adaptations help ectotherms regulate temperature. Diving reptiles conserve heat by heat exchange mechanisms, whereby cold blood from the skin picks up heat from blood moving outward from the body core, re-using and thereby conserving some of the heat that otherwise would have been wasted. The skin of bullfrogs secretes more mucus when it is hot, allowing more cooling by evaporation.^{[citation needed]}

During periods of cold, some ectotherms enter a state of torpor, in which their metabolism slows or, in some cases, such as the wood frog, effectively stops. The torpor might last overnight or last for a season, or even for years, depending on the species and circumstances.

Owners of reptiles may use an ultraviolet light system to assist their pets' basking.