Entropy

Entropy is a macroscopic property of a thermodynamic system that is a measure of the microscopic disorder within the system. It is defined by the second law of thermodynamics. Thermodynamic systems are made up of microscopic objects, e.g., atoms or molecules, which carry energy. According to the second law of thermodynamics, the thermodynamic entropy is a measure of the amount of energy which does no work during energy conversions.

From a thermodynamic point of view, machines are energy conversion devices. Thus, such devices can only be driven by convertible energy. The combination of thermal energy (or its equivalents) and entropy is already converted energy.This is the reason why Rudolf Clausius in 1865 coined the term entropy based on the Greek εντροπία [entropía], a turning toward, from εν- [en-] (in) and τροπή [tropē] (turn, conversion).

The dimension of entropy is energy divided by temperature, and its SI unit is joules per kelvin.