entropy
15.6 Entropy and the Second Law of Thermodynamics: Disorder and the Unavailability of Energy Entropy is the loss of energy available to do work. Another form of the second law of thermodynamics states that the total entropy of a system either increases or remains constant; it never decreases. Entropy is zero in a reversible process; it increases in an irreversible process. The ultimate fate of the universe is likely to be thermodynamic equilibrium, where the universal temperature is constant and no energy is available to do work. Entropy is also associated with the tendency toward disorder in a closed system.