Homeostasis (from Greek: ὅμοιος, hómoios, "similar", and στάσις, stásis, "standing still") is the property of a system that regulates its internal environment and tends to maintain a stable, constant condition of properties such as temperature or pH. It can be either an open or closed system.

It was defined by Claude Bernard and later by Walter Bradford Cannon in 1926, 1929 and 1932.

Typically used to refer to a living organism, the concept came from that of milieu intérieur that was created by Claude Bernard and published in 1865. Multiple dynamic equilibrium adjustment and regulation mechanisms make homeostasis possible.

With regard to any given life system parameter, an organism may be a conformer or a regulator. On one hand, regulators try to maintain the parameter at a constant level over possibly wide ambient environmental variations. On the other hand, conformers allow the environment to determine the parameter. For instance, endothermic animals (mammals and birds) maintain a constant body temperature, while ectothermic animals (almost all other organisms) exhibit wide body temperature variation.