What is meant by isentropic process”?

Answer: The entropy of a gas or fluid is constant is called an isentropic process and it is a thermodynamic process. In this process, neither heat nor matter is not transformed. Because of this, we should only refer to an adiabatic process as isentropic if it is both reversible and adiabatic. 

An idealised version of an actual process, an isentropic process acts as a limiting case for an actual process.

Therefore, reversible adiabatic process is called isentropic process.