The Collaborative International Dictionary of English v.0.48:

Phosphorescence \Phos`phor*es"cence\, n. [Cf. F.
   phosphorescence.]
   1. The quality or state of being phosphorescent.
      [1913 Webster]

   2. The process of phosphorescing, especially that of emitting
      light after a source of excitation has been removed. This
      contrasts with the process of fluorescence, in which a
      substance emits light of a lower wavelength than the
      illuminating light, only while the illumination continues.
      [PJC]

   3. Light emitted by the process of phosphorescence.
      [1913 Webster]

WordNet (r) 3.0 (2006):

phosphorescence
    n 1: a fluorescence that persists after the bombarding radiation
         has ceased