In thermodynamics, a reversible process -- or reversible cycle if the process is cyclic -- is a process whose direction can be "reversed" by means of infinitesimal changes in some property of the system. A reversible process does not increase entropy (of the system and surroundings). During a reversible process, the system is in thermodynamic equilibrium with its surroundings throughout the entire process. Since it would take an infinite amount of time for the reversible process to finish, perfectly reversible processes are impossible. However, if the system undergoing the changes responds much faster than the applied change, the deviation from reversibility may be negligible. In a reversible cycle, the system and its surroundings will be returned to their original states if the forward cycle is followed by the reverse cycle.