In physics, a forbidden mechanism or forbidden line is a spectral line emitted by atomic nuclei, atoms, or molecules undergoing nominally "forbidden" energy transitions not normally "allowed" by the selection rules of quantum mechanics. Obviously, any process which is truly forbidden will not happen. The typical usage of this term, is in a situation where, according to usual approximations (such as the  electric-dipole approximation for the interaction with light), the process cannot happen, but at a higher level of approximation (e.g.  magnetic dipole, or, electric quadrupole) the process is allowed but at a much lower rate.