Reversible and irreversible process

A reversible process is a thermodynamic process that can be reversed by an infinitesimal change in conditions, returning both the system and its surroundings to their initial states without any net change in the universe. It is an idealized process that occurs infinitely slowly, maintaining equilibrium at all times.

An irreversible process is a thermodynamic process that cannot be reversed to restore both the system and its surroundings to their initial states. All real-world processes are irreversible, as they involve a net increase in the entropy of the universe, often due to factors like friction, heat transfer across a finite temperature difference, or rapid expansion.