fluo·res·ce·in 
(fl-rĕsē-ĭn, flô-, flō-)
(fl-rĕsē-ĭn, flô-, flō-)n.
An orange-red compound, C20H12O5, that exhibits intense fluorescence in alkaline solution and is used in medicine for diagnostic purposes, in oceanography as a tracer, and as a textile dye.