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.