Source : Webster's Revised Unabridged Dictionary (1913)

Fluorescein \Flu`o*res"ce*in\, 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.

Source : WordNet®

fluorescein
     n : 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: {fluoresceine}, {fluorescent dye}, {resorcinolphthalein}]