The four laws of thermodynamics define fundamental physical quantities (temperature, energy, and entropy) that characterize thermodynamic systems. The laws describe how these quantities behave under various circumstances, and forbid certain phenomena (such as the perpetuum mobile).

• Zeroth law of thermodynamics : no heat flows between any two bodies at the same temperature.
• First law of thermodynamics : the sum of all energy is constant for all time, but energy can change its type.
• Second law of thermodynamics : the entropy rises steadfastly, because closed systems spontaneously evolve towards thermal equilibrium. The flow of heat from a region of high temperature to a region of low temperature is a spontaneous process.
• Third law of thermodynamics : at zero kelvin all thermal motion ceases.

The four laws were developed during the 19th and early 20th century. Many researchers consider that the zeroth and third laws follow directly from the frist and second laws, thus that there are really only two fundamental laws of thermodynamics.

Thermodynamic entropy is a measure of how evenly energy is distributed in a system. The term was coined in 1865 by the German physicist Rudolf Clausius.

In a physical system, entropy provides a measure of the amount of energy that cannot be used to do work.