Historically, the term Tesla principle was used to describe (amongst other things) certain reversible processes invented by Nikola Tesla. However, this phrase is no longer in conventional use. It was developed during Tesla's research in alternating currents where the current's magnitude and direction varied cyclically.
A reversible process in engineering is a process or operation of a system or device such that a net reverse in operation will accomplish the converse of the original function. The principle was that some systems could be reversed and operated in a complementary manner. During a demonstration of the Tesla turbine, the disks revolved and machinery fastened to the shaft was operated by the engine. If the turbine's operation was reversed, the disks acted as a pump.