In simple terms, the<unk> <unk>  states that the entropy of a pure substance approaches zero as the absolute temperature approaches zero. This<unk>  provides an absolute reference point for the determination of entropy. The entropy determined relative to this point is the absolute entropy.
A special case of this is systems with a unique ground state, such as most crystal lattices. The entropy of a perfect crystal lattice as defined by Nernst's theorem is zero (if its ground state is singular and unique, whereby log(1) = 0).  An example of a system which does not have a unique ground state is one containing half-integer spins, for which time-reversal symmetry gives two degenerate ground states.  Of course, this entropy is generally considered to be negligible on a macroscopic scale.  Additionally, other exotic systems are known that exhibit geometrical frustration, where the structure of the crystal lattice prevents the emergence of a unique ground state.
Real crystals with frozen defects obey this same<unk> , so long as one considers a particular defect configuration to be fixed. The defects would not be present in thermal equilibrium, so if one considers a collection of different possible defects, the collection would have some entropy, but not actually have a temperature. Such considerations become more interesting and problematic in considering various forms of glass, since glasses have large collections of nearly degenerate states, in which they become trapped out of equilibrium.
Another application of the<unk> <unk>  is with respect to the magnetic moments of a material.  Paramagnetic materials (moments random) will order as T approaches 0 K.  They may order in a ferromagnetic sense, with all moments parallel to each other, or they may order in an antiferromagnetic sense, with all moments antiparallel to each other.
Yet another application of the<unk> <unk>  is the fact that at 0 K no solid solutions should exist.  Phases in equilibrium at 0 K should either be pure elements or atomically ordered phases.
Third law of thermodynamics
Second law of thermodynamics
Franck-Condon principle
Planck's law
Hy's law
Chemical thermodynamics
Weber–Fechner law
Equation of state