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A fermion is any particle that has odd-half-integer (1/2, 3/2, ...) intrinsic angular momentum (spin), measured in units of h-bar. As a consequence of this odd-half-integer angular momentum, all fermions obey the Pauli Exclusion Principle.

The fundamental matter particles (quarks and leptons, as well as most composite particles, such as protons and neutrons) are fermions. Therefore, due to the Pauli Exclusion Principle, these particles cannot co-exist in the same location. This is a very important property of ordinary matter!

Next Back For clarification, spin is the internal angular momentum of a particle, in units of h-bar. h-bar = 1.055 x 10^-34 J s.

Look at the Fermion and Boson Chart