In thermodynamics a reversible process (or reversible cycle) is a process in which the overall change in entropy is zero. This defines the boundaries of how efficient a process can be in thermodynamics and engineering; a reversible process is one where no heat is lost from the system as waste, and the process is thus as efficient as it can possibly be.
The second law of thermodynamics states that for any process, the overall change in entropy has to be greater than or equal to zero. For a process where the change in entropy is equal to zero, the process can be run in either direction, and is, hence, reversible.
See also: carnot cycle