Any process that takes place without a change of entropy. The quantity of heat transferred, δ‎Q, in a reversible process is proportional to the change in entropy, δ‎S, i.e. δ‎Q = Tδ‎S, where T is the thermodynamic temperature. Therefore, a reversible adiabatic process is isentropic, i.e. when δ‎Q equals zero, δ‎S also equals zero.