Transitions between energy levels in a quantum-mechanical system that are not allowed to take place because of selection of rules. In practice, forbidden transitions can occur, but they do so with much lower probability than allowed transitions.

The selection rule that is violated is only an approximate value.

When spin - orbit coupling is taken into account, the forbidden transitions become allowed - their strength increasing with the size of the spin orbit coupling; the selection rule is valid for dipole radiation, i.e., in the interaction between a quantum - mechanical system, such as an atom, and an electromagnetic field, only the electric dipole moment is considered.

Actual transitions may involve magnetic dipole radiation or quadrupole radiation; the selection only applies for an atom, molecule, etc. in isolation and does not necessarily apply if external fields, collisions etc., are taken into account.