(1) Literally means: the environment within.
(2) The outside of the cells inside the body of an organism, providing a medium for the fundamental exchange of nutrients and waste.
The term is coined by Claude Bernard, the Father of Physiology. His concept of milieu interieur and the physiological capacity to ensure stability of organs and tissues became the underlying principle of what is known today as homeostasis. The milieu interieur of a multicellular organism is being kept constant and stable through a number of physiological processes occurring within the organism.
Word origin: from the French, milieu de l’intérieur, (the environment within).
Synonym: internal environment
Compare: external environment
See also: homeostasis.