An Etymological Dictionary of Astronomy and Astrophysics

1. A measure of the energy that is not available for work during a → thermodynamic process. It is defined by dS = dQ/T, where dS is the differential change in entropy, dQ is the differential amount of heat introduced to the system in a → reversible process, and T the → absolute temperature of the system. Entropy remains constant during → reversible processes and increases during → irreversible processes without ever decreasing. According to the → second law of thermodynamics, an → isolated system evolves toward a state of maximum entropy. See also → Maxwell’s demon.
   

2. Statistical physics: A measure of → disorder of the configuration of → microstates which make up a → macrostate.
   → Boltzmann’s relation, → Boltzmann’s entropy formula. Highly disordered systems have a large entropy; highly ordered systems have low entropy.
   

3. Math.: A measure of information content. → information entropy.

Etymology (EN): From Ger. Entropie, coined 1865 by physicist Rudolf Clausius (1822-1888) from Gk. entropia “a turning toward,” from en- “in” + trope “a turning, change,” related to tropos “a turn, way, manner,” from tropein “to turn,” from PIE base *trep- “to turn” (cf. L. trepit “he turns”).

Etymology (PE): Dargâšt, from dar “in” + gâšt present stem of gâštan “to cause to revolve, to turn,” transitive of gaštan, variant gardidan “to turn, to change” (Mid.Pers. vartitan; Av. varət- “to turn, revolve;” cf. Skt. vartati; L. vertere; O.H.G. werden “to become;”
PIE base *wer- “to turn, bend”).