Le Chatelier's Principle

One of the better phrasings of Le Chatelier's Priniciple is:

If a stress is applied to a system at equilibrium, then the system readjusts, if possible, to reduce the stress.
M. J. Sienko and R. A. Plane, Chemical Principles and Properties, 2nd ed., McGraw-Hill, New York, NY, 1974, p. 216.

Technically, this principle applies only to chemical reactions. However, experienced practioner's will recognize its applicability to a wide range of domains.

Henri Louis Le Chatelier was a French industrial chemist from the turn of the 20th century (1850-1936). In 1884 he published the following observation:

Any system in stable chemical equilibrium, subjected to the influence of an external cause which tends to change either its temperature or its condensation (pressure, concentration, number of molecules in unit volume), either as a whole or in some of its parts, can only undergo such internal modifications as would, if produced alone, bring about a change of temperature or of condensation of opposite sign to that resulting from the external cause.
H. L. Le Chatelier, Comptes rendus, 99, 786 (1884).

Another version, 1888 is a bit more concise:

Every change of one of the factors of an equilibrium occasions a rearrangement of the system in such a direction that the factor in question experiences a change in a sense opposite to the original change.
H. L. Le Chatelier, Annales des Mines, 13 (2), 157, (1888).