fermion

In physics, a subatomic particle whose spin can only take values that are half-odd-integers, such as 1/2 or 3/2. Fermions may be classified as leptons - particles that do not experience the strong nuclear force, such as the electron - and hadrons - particles that are subject to the strong nuclear force, such as the proton, neutron, mesons, and so on. All elementary particles are either fermions or bosons.

The exclusion principle, formulated by Austrian-US physicist Wolfgang Pauli in 1925, asserts that no two fermions in the same system (such as an atom) can possess the same position, energy state, spin, or other quantized property.