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.