Thermodynamic quantity which is a measure of the degree of disorder within any Definable part of the universe that can be open, closed, or isolated. An open system exchanges both matter and energy with its surroundings. A closed system can only exchange energy with its surroundings; it has walls through which heat can pass. An isolated system cannot exchange energy or matter with. The lower the degree of order, the higher the entropy; for an increase in entropy, DS is positive. Entropy has the units of J/K per mole.