Homeothermic refers to the ability of an organism to maintain a constant body temperature despite external temperature changes. Antonyms for this term are poikilothermic and ectothermic. Poikilothermic organisms have body temperatures that vary with the environment, while ectothermic organisms have body temperatures that are determined by their external environment. Poikilothermic animals include reptiles, amphibians, and fish, while ectothermic animals include insects, arachnids, and some fish. Understanding the different types of thermoregulation in animals is important in studying their behavior, physiology, and evolution.