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 .