law' of thermodynam'ics
1. any of three principles variously stated in equivalent forms, being the principle that the change of energy of a thermodynamic system is equal to the heat transferred minus the work done (first law of thermodynamics), the principle that no cyclic process is possible in which heat is absorbed from a reservoir at a single temperature and converted completely into mechanical work (second law of thermodynamics), and the principle that it is impossible to reduce the temperature of a system to absolute zero in a finite number of operations (third law of thermodynamics).
2. See zeroth law of thermodynamics.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.