Entropy

Statistical physics Entropy is a quantity characterising the disorder of a system. In an irreversible process entropy can only increase (for an isolated system).

SI unit : Joule per kelvin (J.K^-1)

Thermodynamics The entropy of a system is a function of the (extensive) state S, with the differential dS= dQ/T, where dQ is the quantity of heat received reversibly by a thermodynamic system (at temperature T).