A fermion is any particle of matter that is characterized by spin in odd half-integer quantum units (1/2, 3/2, 5/2, etc). Fermions were named by Paul Dirac in 1947 after the Italian physicist Enrico Fermi. They obey the Pauli exclusion principle and Fermi-Dirac statistics, and have antisymmetric wavefunctions.

Quarks, leptons (such as the electron), and baryons (such as the proton and neutron) are all fermions. Compare with boson.