In thermodynamics, a reversible process (or reversible cycle if the process is cyclic) is a process that can be "reversed" by means of infinitesimal changes in some property of the system (Sears and Salinger, 1986).
A process that is not reversible is termed irreversible.
Boundaries and states
A reversible process changes the state of a system in such a way that the net change in the combined entropy of the system and its surroundings is zero. Reversible processes define the boundaries of how efficient heat engines can be in thermodynamics and engineering: a reversible process is one where no heat is lost from the system as "waste", and the machine is thus as efficient as it can possibly be (see Carnot cycle).
In some cases, it is important to distinguish between reversible and quasistatic processes. Reversible processes are always quasistatic, but the converse is not always true. For example, an infinitesimal compression of a gas in a cylinder where there exists friction between the piston and the cylinder is a quasistatic, but not reversible process. Although the system has been driven from its equilibrium state by only an infinitesimal amount, heat has been irreversibly lost due to friction, and cannot be recovered by simply moving the piston infinitesimally in the opposite direction.
Historically, the Tesla principle was the term used for reversible processes.  However, this phrase is no longer in conventional use. The principle was for systems that can be reversed and operate in a complimentary manner. This principle was developed during his Nikola Tesla's research in alternating currents where the current's magnitude and direction varies cyclically. During a demonstration of the Tesla turbine, the disks revolve and machinery fastened to the shaft is operated by the engine. If the turbine's operation is reversed, the disks act as a pump. 
1. ^ Sears, F.W. and Salinger, G.L. (1986), Thermodynamics, Kinetic Theory, and Statistical Thermodynamics, 3rd edition (Addison-Wesley.)
2. ^ Giancoli, D.C. (2000), Physics for Scientists and Engineers (with Modern Physics), 3rd edition (Prentice-Hall.)