Phosphorescence is a similar phenomenon to fluorescence. However, instead of the electron returning to its original energy state following excitation, it decays to an intermediate state (with a different spin value) where it can remain for some time (minutes or even hours) before decaying to its original state. Phosphorescence is one form of Luminescence, especially Photoluminescence.
Reference: Wikipedia (2011): “Phosphorescence”.
MitoPedia methods: Fluorometry, Spectrophotometry