[ General Glossary, Quantum Glossary, Atomic Glossary ]
Permitted, Forbidden & Semi-Forbidden Transitions

In common astronomical usage,

Permitted Transitions (or "Allowed")	are Electric Dipole (E1) transitions for which all the "standard" selection rules apply.
Semi-Forbidden Transitions	are Electric Dipole (E1) transitions but for which the selection rule that S does not change is violated A single right bracket (]) is usually used in the literature to designate such a transition eg CIII]
Forbidden Transitions	are transitions which violate one of the other selection rules for Electric Dipole (E1) transitions (and hence include Magnetic Dipole (M1), Electric Quarupole (E2), etc transitions. A pair of brackets ([]) is usually used in the literature to designate such a transition eg [OIII]