The pressure in a degenerate gas of fermions caused by the Pauli exclusion principle and the Heisenberg uncertainty principle. Because of the exclusion principle, fermions at a high density, with small interparticle spacing, must have different momenta; from the uncertainty principle, the momentum difference must be inversely proportional to the spacing. Consequently, in a high-density gas (small spacing) the particles have high relative momenta, which leads to a degeneracy pressure much greater than the thermal pressure. White dwarfs and neutron stars are supported against collapse under their own gravitational fields by the degeneracy pressure of electrons and neutrons, respectively. This pressure is sometimes called the Fermi pressure.