Pauli Exclusion Principle Definition:

The Pauli’s exclusion principle states that for an atomic orbital to occupy a maximum of two electrons, the electrons must have an opposite spin.

Pauli Exclusion Principle Explained:

From Quantum theory, we understand that each electron has a given set of 3 spatial quantum numbers given by the solution to the Schrödinger wave equation. However, there is also a fourth quantum number that defines the spin of the electron which is an intrinsic property of the electron. The Pauli’s exclusion principle, therefore, refers to the finding that in order for an atomic orbital to contain 2 electrons, both the electrons must have an opposite spin, all other spacial quantum numbers being the same.