1. : a law in physics: 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