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An elementary or composite particle, such as an electron, quark, or proton, whose spin is an integer multiple of 1/2 . Fermions act on each other by exchanging bosons and are subject to the Pauli exclusion principle, which requires that no two fermions be in the same quantum state. Fermions are named after the physicist Enrico Fermi, who along with Paul Dirac developed quantum statistical models of their behavior. Compare boson.