Definition of "isentropic" [i•sen•trop•ic]

• Without change in entropy; at constant entropy. (adjective)

American Heritage(R) Dictionary of the English Language, Fifth Edition. Copyright (c) 2011 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.

• "Some spontaneous macroscopic evolutions are reversible and isentropic, and an expanding radiation-dominated universe is one example."
• "The only reasonable conclusion that respects the Second Law is that the expansion of a radiation-only universe is reversible and so isentropic in all phases."
• "If you move the walls of a gas-filled 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."