In thermodynamics, a cycle refers to the process by which a working body undergoes a series of transformations in which its final state results to be identical with its initial state. [1] The concept of the "cycle" was first outlined by French physicist Sadi Carnot in 1824. [2]

References
1. Perrot, Pierre. (1998). A to Z of Thermodynamics, Oxford: Oxford University Press.
2. (a) Carnot, Sadi. (1824). “Reflections on the Motive Power of Fire and on Machines Fitted to Develop that Power.” Paris: Chez Bachelier, Libraire, Quai Des Augustins, No. 55.
(b) Thims, Libb. (2007). Human Chemistry (Volume One), (ch. 3: Carnot's Cycle, pgs. 37-71), (preview), (Google books). Morrisville, NC: LuLu.