White dwarfs and Neutron stars exist due to the "degeneracy" type of this.
When electrons are moving at relativistic speeds, one form of this quantity is related to the number density of the electrons to the 4/3rds power. For electrons moving at non-relativistic speeds, that form is related to h-bar squared times the number density of the electrons to the 5/3rds over the mass of an electron. Another type of this quantity is equal to the electric field crossed with the magnetic field over the speed of light, or the (*) Poynting vector over the speed of light. White dwarfs and Neutron stars exist due to the "degeneracy" type of this. One "force" caused by this quantity is equal to negative one over this quantity times the gradient of this quantity and is responsible for winds. When an object is in hydrostatic equilibrium, this quantity's gradient force counterbalances gravity. Isobars represent equal values of this quantity that is equal to nkT according to the ideal gas law. For 10 points, name this physical quantity that is equal to force over area, measured in atmospheres, and related by Boyle's law to volume.
When electrons are moving at relativistic speeds, one form of this quantity is related to the number density of the electrons to the 4/3rds power.
pressure [accept electron degeneracy pressure, Fermi pressure, etc.]