Definitions
from The American Heritage® Dictionary of the English Language, 4th Edition
 adj. Without change in entropy; at constant entropy.
from Wiktionary, Creative Commons Attribution/ShareAlike License
 adj. having a constant entropy
from the GNU version of the Collaborative International Dictionary of English
 adj. Having equal entropy.
from The Century Dictionary and Cyclopedia
 In physics, of equal entropy.
 n. An isentropic line: usually in the plural, isentropics.
from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.
 adj. with unchanging entropy; at constant entropy
Etymologies
Examples

Some spontaneous macroscopic evolutions are reversible and isentropic, and an expanding radiationdominated universe is one example.

The only reasonable conclusion that respects the Second Law is that the expansion of a radiationonly universe is reversible and so isentropic in all phases.

If you move the walls of a gasfilled box to expand its volume, and do it (arbitrarily) slowly enough that the gas remains (arbitrarily) close to equilibrium throughout, then the expansion is isentropic and reversible.

If we restrict such a universe to contain _only_ radiation, then its expansion and recontraction are basically reversible and isentropic (I argued this back in the “Latest Declamations About the Arrow of Time” post, and you moreorless agreed; see comments #60, 61 and 67 in that thread).

A technical process like an engine process involves losses. heat transfer and other irreversibilities and cannot therefore be described by the isentropic exponent g.

The isentropic exponent g is a specific constant of a gas or a gas mixture and is defined as

· Direct calculation of steam outlet conditions for isentropic, isenthalpic, constant internal energy or constant volume process

Cv, also sometimes called the adiabatic index or the heat capacity ratio or the isentropic expansion factor

The details are a bit hairier as they involve isentropic lifting this morning, an upperlevel disturbance later today and finally the approach of a cold front this evening.

The working fluid is air, there is no friction, the compression and expansion are isentropic, the air temperature does not change during the heat addition and heat rejection, the air behaves as an ideal gas (i.e., pv = RT) and the specific heat is constant.
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