in physics, a particular configuration, usually reached at high densities, of a gas composed of subatomic particles with half-integral intrinsic angular momentum (spin). Such particles are called fermions (fermion), because their microscopic behaviour is regulated by a set of quantum mechanical rules— Fermi-Dirac statistics (q.v.). These rules state, in particular, that there can be only one fermion occupying each quantum-mechanical state of a system. As particle density is increased, the additional fermions are forced to occupy states of higher and higher energy, because the lower-energy states have all been progressively filled. This process of gradually filling in the higher-energy states increases the pressure of the fermion gas, termed degeneracy pressure. A fermion gas in which all the energy states below a critical value (designated Fermi energy) are filled is called a fully degenerate, or zero-temperature, fermion gas. Such particles as electrons, protons, neutrons, and neutrinos are all fermions and obey Fermi-Dirac statistics. The electron gas in ordinary metals and in the interior of white dwarf stars constitute two examples of a degenerate electron gas.