Rudolf Clausius introduced the term entropy in 1865.
Clausius had developed the underlying thermodynamic idea during the 1850s, while analyzing why heat engines cannot convert all supplied heat into useful work. In his 1865 paper, he named the quantity entropy, using a Greek-derived word associated with transformation. He also gave entropy its familiar symbol, S.
In classical thermodynamics, entropy is a state function. For a reversible transfer of heat, its change is defined by dS = δQrev/T, where T is absolute temperature. This made it possible to describe the direction of natural processes mathematically and to express the second law: the entropy of an isolated system does not decrease.
A common mix-up is to credit Ludwig Boltzmann with inventing entropy. Boltzmann later supplied its statistical interpretation, linking entropy to the number of microscopic arrangements available to a system. Clausius introduced the thermodynamic concept and the name; Boltzmann deepened its microscopic meaning.