A theorem that arises from
Quantum Field Theory. It shows that any quantum entity (
particles,
atoms, etc.) can be classified by it's total
spin angular momentum. If its spin is equal to 1/2 of a positive odd integer it is called a
fermion and obeys
Fermi-Dirac Statistics and hence obeys the
Pauli Exclusion Principle. If its spin is equal to a positive integer it is called a
boson and obeys
Bose-Einstein Statistics.
The Spin-Statistics theorem is one of the most important results of modern physics, but it is also one of the hardest to prove or understand.