In thermodynamics, the second main principle, in contrast to the first main principle, of the mechanical theory of heat is defined by the following two equivalent expressions: [1]

$\frac{dQ}{T}=dS$

which can be rearranged as:

where dQ is an amount of heat imparted to a body, T is the temperature of the body at the point of heat transfer, and dS is the change in the transformation content of the body or of entropy change. These expressions were the formulations used by German physicist Rudolf Clausius, which he characterized as "convenient in the case of certain investigations", later coming to be known as the second law of thermodynamics. Clausius also called this the “principle of equivalence of transformations”, another formulation of what later came to emerge as the second law.

References
1. Clausius, Rudolf. (1879). The Mechanical Theory of Heat (pg. 90; 91-109), London: Macmillan & Co.