Pauli Exclusion Principle
Definition
No two electrons in an atom may share the exact same quantum address — the rule that builds the entire periodic table.
Technical definition
Fermion wavefunctions are antisymmetric under particle exchange, so two identical fermions cannot occupy the same quantum state.
Example
White dwarf stars resist gravitational collapse through electron degeneracy pressure — Pauli exclusion refusing to let electrons share states.
Common confusion
Pauli exclusion applies to fermions (electrons, protons, neutrons) — not to bosons. Photons happily pile into the same state, which is exactly how lasers work.