The Collaborative International Dictionary of English v.0.48:

fluorescein \fluo`res"ce*in\, fluoresceine \fluo`res"ce*ine\,
fluorescein \flu`o*res"ce*["i]n\, n. (Chem.)
   A yellowish red, crystalline substance, C20H12O5, produced
   by heating together phthalic anhydride and resorcin; -- so
   called, from the very brilliant yellowish green fluorescence
   of its alkaline solutions. It has acid properties, and its
   salts of the alkalies are known to the trade under the name
   of uranin.

   Syn: fluorescein, fluorescent dye, resorcinolphthalein.
        [1913 Webster]

WordNet (r) 3.0 (2006):

fluoresceine
    n 1: a yellow dye that is visible even when highly diluted; used
         as an absorption indicator when silver nitrate solution is
         added to sodium chloride in order to precipitate silver
         chloride (turns pink when no chloride ions are left in
         solution and negative fluorescein ions are then absorbed)
         [syn: fluorescein, fluoresceine, fluorescent dye,
         resorcinolphthalein]