In thermodynamics, the second law of thermodynamics quantifies the energetic nature of irreversibility in heat driven transformation processes, particularly in the Carnot cycle. In concise form, the second law, as formulated mathematically in 1862 by German physicist Rudolf Clausius, states that in a cyclical heat-driven process which is in any way possible the following relation will always hold:

where dQ is an element of the heat given up by a body to any reservoir of heat during its own changes, heat which it may absorb from a reservoir being here reckoned as negative, and T is the absolute temperature of the body at the moment of giving up this heat. [1] The quantity "dQ/T" is called entropy.

References
1. (a) Clausius, Rudolf. (1862). "On the Application of the Theorem of the Equivalence of Transformations to Interior Work", (pp. 215-250).
(b) Clausius, R. (1865). The Mechanical Theory of Heat – with its Applications to the Steam Engine and to Physical Properties of Bodies. London: John van Voorst, 1 Paternoster Row. MDCCCLXVII.
(c) Clausius, Rudolf. (1879). The Mechanical Theory of Heat (second edition). London: Macmillan & Co.