thermodynamic parameter entropy A thermodynamic property which acts as a measure of the state of disorder of a system. Its natural variables are the internal energy (U) and the volume (V). It is defined by dS = (1/T)dU + (p/T)dV. The second law of thermodynamics states that in an isolated system, natural processes tend to increase in disorder or entropy. The standard unit of measure is Joules per Kelvin (J/K). symbol: S