metastable state

in spectrochemistry
Any excited state which in principle, by virtue of the selection rules, cannot radiatively combine with any lower state. It usually has a considerably longer lifetime than an ordinary excited state.
PAC, 1985, 57, 1453. 'Nomenclature, symbols, units and their usage in spectrochemical analysis - V: Radiation sources (Recommendations 1985)' on page 1463 (