Nernst's theorem, sometimes referred to as the third law of thermodynamics, states that the entropy of a system at zero absolute temperature is a well-defined constant. This is due to the fact that a system at zero temperature exists in its ground state, so that its entropy is determined by the degeneracy of the ground state. Many systems, such as crystal lattices, have a unique ground state, and therefore have zero entropy at at absolute zero (since ln(1) = 0).
