WordNet (r) 3.0 (2006):

resorcinolphthalein
    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]