A process is reversible when it can be reversed, leaving no trace of the original process and returning the system to its original [[State|state]]. Reversible processes are nearly impossible in real life due to factors such as [[Friction|friction]], [[Mixing|mixing]], and [[Heat|heat]] transfer. The reversible process is considered the theoretical ideal process, acting as the limiting case for the [[Efficiency|efficiency]] of a [[Cycle|cycle]]. Any cycle must have an efficiency less than or equal to the efficiency of the reversible version of that cycle.