entropy

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun For a closed thermodynamic system, a quantitative measure of the amount of thermal energy not available to do work.
• noun A measure of the disorder or randomness in a closed system.
• noun A measure of the loss of information in a transmitted message.
• noun The tendency for all matter and energy in the universe to evolve toward a state of inert uniformity.
• noun Inevitable and steady deterioration of a system or society.

from The Century Dictionary.

• noun In physics: As used by Clausius, the inventor of the word, and others, that part of the energy of a system which cannot be converted into mechanical work without communication of heat to some other body, or change of volume.
• noun As used by Tait and others, the available energy; that part of the energy which is not included under the entropy in sense .

from the GNU version of the Collaborative International Dictionary of English.

• noun (Thermodynamics) A certain property of a body, expressed as a measurable quantity, such that when there is no communication of heat the quantity remains constant, but when heat enters or leaves the body the quantity increases or diminishes. If a small amount, h, of heat enters the body when its temperature is t in the thermodynamic scale the entropy of the body is increased by h ÷ t. The entropy is regarded as measured from some standard temperature and pressure. Sometimes called the thermodynamic function.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun statistics, information theory, countable A measure of the amount of information and noise present in a signal. Originally a tongue in cheek coinage, has fallen into disuse to avoid confusion with thermodynamic entropy.
• noun uncountable The tendency of a system that is left to itself to descend into chaos.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun (communication theory) a numerical measure of the uncertainty of an outcome
• noun (thermodynamics) a thermodynamic quantity representing the amount of energy in a system that is no longer available for doing mechanical work