Metastable electron by Ciro Santilli 34 Updated Created
A fundamental component of three-level lasers.
As mentioned at youtu.be/_JOchLyNO_w?t=581 from Video "How Lasers Work by Scientized (2017)", they stay in that state for a long time compared to non spontaneous emission of metastable states!
This is also one of mechanisms behind phosphorescence with triplet states.