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 = 0, δS also equals 0.
Subjects: Chemistry — Physics.