Which law is often called the 'law of increasing entropy'?

The second law of thermodynamics is often called the law of increasing entropy. It describes the direction in which natural processes tend to occur.

Entropy measures, in thermodynamics, how energy and microscopic states are distributed. For an isolated system, total entropy cannot decrease: it remains constant in an ideal reversible process and increases in an irreversible one. Heat flowing through a temperature difference, gas expanding into a vacuum, and friction converting organized motion into thermal energy are familiar examples.

The phrase does not mean that every individual part of every system must become more disordered. A refrigerator can create a more organized, colder interior, but it does so by consuming work and increasing the entropy of its surroundings by an even greater amount. The total entropy change remains nonnegative.

The other laws have different roles. The first law concerns conservation of energy, the zeroth law defines thermal equilibrium, and the third law describes entropy near absolute zero. The second law adds the crucial direction-of-change rule.