The third law of thermodynamics links entropy approaching a constant minimum near 0 K.
As a system’s temperature approaches absolute zero, its entropy approaches a limiting value. For a perfect crystal with a unique ground state, that value is conventionally zero. Entropy measures the number of microscopic configurations compatible with a macroscopic state, so a unique ground state leaves no configurational uncertainty at absolute zero.
The law was developed chiefly through the work of Walther Nernst between 1906 and 1912 and is often associated with the Nernst heat theorem. It provides an absolute reference point for calculating entropies at higher temperatures and also implies the unattainability principle: no finite sequence of physical operations can cool a system to exactly 0 K.
The minimum is not always exactly zero. Materials with multiple ground states, structural disorder, or glassy behavior can retain residual entropy at 0 K. This is why the careful wording is “a constant minimum” rather than an unconditional claim that every substance has zero entropy there. The second law instead concerns entropy production and the direction of spontaneous processes.