forbidden
adjective
                                                                                                                            
                                                            for·bid·den
                    
                                                                                                              
                                                                                                                                  fər-ˈbi-dᵊn  
                                                                                                                              
            fȯr-
                                                      
                                                          1
                    
                                          
              
          
                                                      : not permitted or allowed                                      
                
                    
2
                    
                                          
              
          
                                                      : not conforming to the usual selection principles                                      
                
                  —used of quantum phenomena
forbidden transition
forbidden radiation
forbidden spectral line
    
                
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  Merriam-Webster unabridged




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