∞ generated and posted on 2016.08.29 ∞
The idea that living systems, in order to survive and prosper, must control within relatively narrow ranges various physical, chemical, and physiological parameters associated with their bodies.
A basic underlying principle of homeostasis is that of negative feedback where changes in parameters within a system, e.g., temperature, result in the invocation of forces that drive the system back towards some preset "normal" state.
Note that different organisms attempt to control different parameters, with some controlling more things and others less. More control, though, generally requires greater inputs of energy than less or lack of control, though a lack of homeostatic control over important parameters can be more limiting in terms of what kinds of environments an organism is capable of surviving and prospering in.
Homeostasis is sort of the biological equivalent of for every action there is an equal and opposite reaction. That is, for every change away from "normal" the body reacts by invoking mechanisms that drive parameters back towards "normal".
When your body is cold, for example, it attempts (in various ways) to warm itself up.