Processes are classified as reversible or irreversible based on whether they increase or keep constant the total entropy of the system. Processes that cause an increase in the net entropy of the system are irreversible, while those that keep the entropy constant are reversible.
Entropy is an indication of system randomness. As all systems tend to the most random and homogeneously distributed state, systems that undergo randomizing processes cannot be reversed without external intervention. If a process requires external intervention to be reversed, it is irreversible. The discharge of a rechargeable battery is an irreversible process, even if the battery is rechargeable, because once the battery is discharged, it would never recharge itself without external intervention, such as the input of electric power from an outside source.
Reversible processes include reactions that proceed at equal rates in the forward and backwards directions at equilibrium, such as ionic dissolution and reformation reactions, or the hypothetical restrained compression and expansion of a gas. Left alone, reversible processes can continue to proceed in the forward and backwards reaction directions indefinitely. Although mechanical, reversible processes are theoretically possible, they are impossible to realize because of the presence of irreversible dissipation of heat due to friction.