Poikilotherm: An animal that varies its internal body temperature within a wide range of temperatures, usually as a result of variation in the environmental temperature.

Homeotherm: An animal that maintains a constant internal body temperature, usually within a narrow range of temperatures.

Homeostasis is the body's ability to maintain a stable internal environment despite external changes, encompassing processes like temperature regulation, blood sugar control, and fluid balance. 

Osmoregulation is the active regulation of an organism's body fluids to maintain the homeostasis of its water content and salt balance, essential for metabolic activities and survival. 

The body temperature regulation of a lizard is best described as poikilothermy (or ectothermy), as their body temperature fluctuates with the environment, unlike the homeothermic regulation seen in mammals and birds.