Residual entropy is entropy remaining in the zero-temperature limit. An equilibrium ground-state degeneracy gives for the complete system; an extensive residual entropy requires a correspondingly large state count. Frozen configurational disorder can also leave an experimentally inferred residual entropy when a sample fails to equilibrate. The zero-entropy perfect-crystal form of the Third law of thermodynamics assumes a unique, fully ordered equilibrium ground state.
New to topics? Read the docs here!