The term for when an organism's body temperature fluctuates with its environment is called ectothermy or poikilothermy. Ectothermic organisms, such as reptiles and amphibians, rely on external heat sources to regulate their body temperature. This adaptation allows them to conserve energy but also makes them dependent on environmental conditions. Unlike endothermic organisms, which maintain a constant body temperature, ectotherms often experience a range of physiological responses depending on their surroundings.