(4) Entropy

Heat flows from a substance at a high temperature to a substance at a low temperature, and does not flow reversibly (The 2nd low of thermodynamics).
(Entropy is derived from the 2nd law of thermodynamics, and is a quantity of state that is represented by ds=dq/T)
Entropy is what shows how much difference there is in a change from a reversible change.
A reversible change is a change where energy transfer with its surrounding returns as it was at the beginning when the change is reversed.
For example, when a compressor with an adiabatic compression is driven by a refrigerant , it becomes an adiabatic expansion engine.
This is the adiabatic change. When a change is adiabatic, entropy does not change.
In addition, an adiabatic change is not always a reversible change.

Entropy is used in a compression process in a refrigeration cycle.
During an ideal compression process, an adiabatic compression is achieved where no heat transfer occurs between a refrigerant and a cylinder and there is no change in entropy. Therefore, the compression process is drawn parallel to an isentropic line on a p-h diagram.

NOTE: An expansion process is also an adiabatic change, but is not a reversible change. It is because an substance flows from a high pressure to a low pressure and does not flow reversibly. --- This is also another expression of the second law.
An evaporation process and a condensation process are non-reversible change and entropy increases.