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I am looking for a definition for ISENTROPIC that in plain ordinary English.
Can you help?

Answering your question in "plain ordinary English" is quite a challenge.  Probably because ISENTROPIC refers to a process that occurs at constant ENTROPY which is a "measure of disorder".   ENTROPY is a difficult concept for anyone to understand.  Consider this:

Let's say you are in a hot air ballon 100 feet above the ground.  You drop
100 bricks to the ground.  The likelyhood of the brick landing one on top of
another to form a brick wall is very, very small.  However, the chances of
the bricks landing in a jumbled pile on the ground are very good.  We would
say that the "LIKELYHOOD OF THE JUMBLED PILE" is higher than that of the
brick wall or the ENTROPY OF THE JUMBLED PILE IS HIGHER than the brick wall.

Essentially, the greater the statistical probability of a particular state or arrangement of "things", the greater the entropy.  For example, a
crystalline solid of highly order molecules (such as salt) has a much LOWER entropy (degree of disorder) and a pool of liquid water.  The liquid water molecules are much more DISORDERED than the crystalline structure of the salt.

ISENTROPIC would be like saying that the chances the bricks falling to the ground and forming a wall are EXACTLY EQUAL to the chances of the bricks walling into a disorder pile.

To my knowledge, real world examples of ISENTROPIC systems are rare...in fact I can't think of one!  It is sometimes used to analyze systems of gases when assuming ideal gas properties (perfectly elastic molecules collisions).  This is good for estimating and reduces the difficulty of the problems without sacraficing significant accuracy.   However, assuming ideal gas properties is generally safe only at lower pressures.

Hope I've helped here...."plain English"?.....I'm not sure....but I think it's as close as you'll get!  Good Luck!