2 Physics denoting or involving a transition between two quantum-mechanical states that does not conform to some selection rule, especially for electric dipole radiation.
Phrases
the forbidden degrees
the number of steps of descent from the same ancestor that bar two related people from marrying.
forbidden fruit
a thing that is desired all the more because it is not allowed.
[with biblical allusion to Gen. 2:17]
forbidden in other Oxford dictionaries
Definition of forbidden in the US English dictionary
