What is the name of the principle that states that no two fermions can occupy the same quantum state?

The principle stating that no two identical fermions can occupy the same quantum state is the Pauli exclusion principle.

Austrian physicist Wolfgang Pauli formulated the principle in 1925 while studying the structure of electrons in atoms. An electron’s quantum state is described by quantum numbers, including its energy level, orbital shape, orbital orientation, and spin. Two electrons may share an orbital, but they must have opposite spins, so their complete quantum states are different.

The principle explains why electrons fill distinct shells and why atoms have their varied chemical properties. It also helps explain the stability of ordinary matter: electrons cannot all fall into the lowest-energy state. On much larger scales, electron degeneracy pressure supports white dwarfs against gravitational collapse.

The most common mistake is applying the rule to all particles. Fermions, such as electrons, protons, neutrons, and quarks, obey it. Bosons, such as photons, do not; many identical bosons can occupy one quantum state, as in a laser or Bose–Einstein condensate.