- exclusion principle.
- physics the principle that two identical fermions cannot occupy the same quantum state in a body such as an atomSometimes shortened to: exclusion principle
- The principle that two fermions of a given type, such as electrons, protons, or neutrons, cannot occupy the same quantum state. It does not apply to bosons. This principle plays a key role in the electron orbital structure of atoms, since it prevents more than two electrons from occupying any given orbital (two are allowed, since they may have opposite spin, and thus be in different quantum states). See also orbital degeneracy pressure.