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1. A measure of the extent to which the energy of a system is unavailable. A mathematically defined thermodynamic function of state, the increase in which gives a measure of the energy of a system which has ceased to be available for work during a certain process:

ds = (du + pdv)/T >= dq/T
where s is

specific entropy; u is specific internal energy; p is pressure; v is specific volume; T is Kelvin temperature; and q is heat per unit mass. For reversible processes,

ds = dq/T
In terms of potential temperature Missing Image:img src="SP7-e_files/thetasm.gif",
ds = cp (dMissing Image:img src="SP7-e_files/thetasm.gif"/Missing Image:img src="SP7-e_files/thetasm.gif")
where cp is the

specific heat at constant pressure. See third law of thermodynamics. </dd>
In an adiabatic process, the entropy increases if the process is irreversible and remains unchanged if the process is reversible. Thus, since all natural processes are irreversible, it is said that in an isolated system the entropy is always increasing as the system tends toward equilibrium, a statement which may be considered a form of the second law of thermodynamics. </dd>
2. In communication theory, average information content. </dd>


This article is based on NASA's Dictionary of Technical Terms for Aerospace Use