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: Webster's Revised Unabridged Dictionary (1913)