In particle physics, a fermion is a particle that follows Fermi-Dirac statistics and generally has half-integer spin: spin 1/2, spin 3/2, etc. These particles obey the Pauli exclusion principle. Fermions include all quarks and leptons, as well as all composite particles made of an odd number of these, such as all baryons and many atoms and nuclei. Fermions differ from bosons, which obey Bose-Einstein statistics.
Named after Enrico Fermi, who developed the statistical rules governing them. The term was coined by Paul Dirac.
Fermions are the building blocks of matter (electrons, protons, neutrons are all fermions), while bosons are the force carriers.