Ectotherm

From UroWiki

An endothermic creature (such as mammals, inclduing human beings) can control its body temperature using internal regulation, such as increasing the diameter of its capillaries and sweating to cool down. An ectothermic creature like a reptile must use external regulation, such as sitting in the sun (or, in captivity, under a basking lamp) to warm up its body. This is why the correct basking spot temperatures, coupled with an appropriate thermal gradient are extrememly important for your lizard.