Pauli exclusion principle
The Pauli exclusion principle is a fundamental rule in subatomic physics which states that no two electrons, or other types of fermion (governed by Fermi-Dirac statistics), can occupy the same quantum state at the same place and time. It was formulated by the Austrian-born physicist Wolfgang Pauli in 1925. In a system of fermions, the lack of empty neighboring states often prevents most particles from contributing to the system properties, which thus depend only on the states bordering the filled ones – the so-called Fermi surface.
The exclusion principle prevents white dwarfs that are below the Chandrasekhar limit from collapsing: the electrons are squashed together to form electron degenerate matter, which resists any further attempts at compression.