TIA's Glossary of Telecommunication Terms
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transition frequency
The frequency associated with the difference between two discrete energy levels in an atomic system, given by 
where f 2,1 is the frequency associated with the difference between two energy levels, E 2 and E 1 (E 2> E 1), and is Planck's constant. Note: If a transition from E 2 to E 1 occurs, a photon with frequency f 2,1 is likely to be emitted. If the atomic system is at energy level E 1, and a photon of frequency f 2,1 is absorbed, the energy level will be raised to E 2. [From Weik '89]