Definition of Pauli Exclusion Principle
Pauli exclusion principle dictates that the two electrons present in an atom or in a molecule can never have the same four electronic quantum numbers. An orbital can accommodate a maximum of two electrons and these two electrons must spin in opposite directions to minimize the repulsive forces. If one electron has a clockwise spin, the other must be spinning in a counterclockwise direction. The electrons present in the same orbital have their first three quantum numbers same.
Example of Pauli Exclusion Principle:
Take an example of 1s orbital which is having one electron in it, the electron configuration would be written as 1s1 and if there are two electrons, then the electron configuration would be 1s2.