law of thermodynamics

1 : conservation of energy

— called also first law of thermodynamics

2 : a law in physics: mechanical work can be derived from the heat in a body only when the body is able to communicate with another at a lower temperature or all actual spontaneous processes result in an increase of total entropy

— called also second law of thermodynamics

3 : a law in physics: at the absolute zero of temperature the entropy of any pure crystalline substance is zero and its derivative with respect to temperature is zero

— called also third law of thermodynamics