Homeostasis is the coordinated set of physiological mechanisms that preserve a stable internal bodily environment (through feedback-controlled regulatory processes) despite fluctuations in the external environment. The concept applies to many dynamic regulations of physiological variables such as body temperature, pH, electrolyte balance, osmolarity, blood glucose, etc.
Claude Bernard (1813–1878) introduced the concept of a “milieu intérieur” the “internal environment” in 1865 where he stated “La fixité du milieu intérieur est la condition de la vie libre et indépendante.” (The constancy of the internal environment is the condition for a free and independent life). Walter B. Cannon (1871–1945) formally coined the term homeostasis in 1929. In his reasoning to name these processes under one name he used the Greek term "ομοιόσταση" [omeóstasí) meaning "constant & stable" or "similar & standing still" referring to a constant (internal) environment.
Disruption of homeostatic processes can contribute to disease states such as diabetes mellitus (failure of glucose homeostasis), heart failure (impaired circulatory stability), heat stroke and/or hypothermia (thermoregulatory failure), hyponatremia or hypernatremia (electrolyte imbalance), etc.
For additional information here is an article from the National Library of Medicine.
Sources:
1. Claude Bernard, "Introduction à l’étude de la médecine expérimentale" (1865).
2. “Organization for physiological homeostasis” Cannon W. B.; Physiol Rev. 1929; 9:399–431.
3. "Textbook of Medical Physiology"; Guyton, Arthur C and Hall, John E ISBN: 0721659446 USA: W.B. Saunders, 1996.
4. “Homeostasis and Body Fluid Regulation: An End Note”. De Luca LA Jr, David RB, Menani JV. Neurobiology of Body Fluid Homeostasis: Transduction and Integration. Boca Raton (FL): CRC Press/Taylor & Francis; 2014. Chapter 15