Phosphorescence is a kind of luminescence (i.e., a kind of light emission of a medium) which lasts relatively long after excitation of the medium (for example many milliseconds or more). The excitation energy is stored in metastable electronic states (often triplet states reached via intersystem crossings), exhibiting only forbidden transitions to lower states. There are also cases with delayed fluorescence, where one has an intersystem crossing to a triplet state and later on back to the original singlet state. Very long decay times can also occur due to thermally activated delayed fluorescence (TADF), where an involved thermal excitation is very slow because of an energy gap much larger than kB T. Some authors would not call that phosphorescence, although it is common to call certain substances phosphorescent where such traps play a vital role.
As the stored energy can be released only through relatively slow processes, phosphorescence is generally much weaker than fluorescence.
Important phosphorescent materials are europium-doped strontium aluminate (Eu2+:SrAl2O4) and zinc sulfide (Eu2+:ZnS). They are used e.g. in safety products such as exit signs, which are visible even under conditions of power failure, although they are not very bright. Phosphorus, from which the name phosphorescence was originally derived, actually exhibits chemoluminescence.