Residual entropy is the difference in
entropy between a non-equilibrium state and crystal state of a substance close to
absolute zero. This term is used in
condensed matter physics to describe the entropy at zero kelvin of a
glass or
plastic crystal referred to the crystal state, whose entropy is zero according to the
third law of thermodynamics. It occurs if a material can exist in many different states when cooled. The most common non-equilibrium state is vitreous state,
glass.