Any of a group of subatomic particle s having odd half-integral spin (12, 32).
Fermions are named for the Fermi-Dirac statistics that describe their behaviour. They include particles in the class of lepton s, baryon s, and nuclei of odd mass number (e.g., tritium, helium-3, uranium-233). They obey the Pauli exclusion principle . Fermions are produced and undergo annihilation in particle-antiparticle pairs. See also boson .