A theorem stating that, for a thermodynamic system exchanging heat with external reservoirs and undergoing a thermodynamic cycle, ∮(𝛿Q)/(T_surr)≤0,, where 𝛿Q is the infinitesimal amount of heat absorbed by the system from the reservoir, and T_surr is the temperature of the external reservoir (surroundings) at a particular instant in time.

A way of characterizing a discontinuous phase transition between two phases of matter of a single constituent. It gives the slope of the tangents to the "coexistence curve" that separates the two phases on a pressure-temperature diagram.