Discussions by Yann Oliver
Homoiothermy means that an animal has a (roughly) constant internal temperature. The opposite is poikilothermy.
Endothermy means the ability for an animal to produce itself its internal heat in order to get a constant temperature. On the contrary, ectothermy means that the animal gets its heat from its environment.
Mass homoiothermy (or inertial homoiothermy, or gigantothermy) is a kind of non-endothermic homoiothermy proposed as a possible metabolism for dinosaurs. Its basic idea is that the heat exchange with the environment is due to the surface of the skin (proportional to its area), whereas the total amount of heat is proportional to the body mass. This means that a big animal proportionally loses less heat than a small one (the area is proportional to the square of the size, and the mass, to its cube).
Thus, big animals like some dinosaurs can maintain a constant body temperature simply by inertia, with occasional exposition to solar heat, without having to produce their heat by themselves.