Source : Webster's Revised Unabridged Dictionary (1913)

Fluorescence \Flu`o*res"cence\, n.
   A property possessed by fluor spar, uranium glass, sulphide
   of calcium, and many other substances, of glowing without
   appreciable rise of temperature when exposed to light or to
   ultra-violet rays, cathode rays, X rays, etc.

Fluorescence \Flu`o*res"cence\, n. [From {Fluor}.] (Opt.)
   That property which some transparent bodies have of producing
   at their surface, or within their substance, light different
   in color from the mass of the material, as when green
   crystals of fluor spar afford blue reflections. It is due not
   to the difference in the color of a distinct surface layer,
   but to the power which the substance has of modifying the
   light incident upon it. The light emitted by fluorescent
   substances is in general of lower refrangibility than the
   incident light. --Stockes.

Source : WordNet®

fluorescence
     n : light emitted during absorption of radiation of some other
         (invisible) wavelength