Definition of PHENOLPHTHALEIN. : a white or yellowish-white crystalline compound C20H14O4 used in analysis as an indicator because its solution is brilliant red in alkalies and is decolorized by acids and in medicine as a laxative. See phenolphthalein defined for kids.ADVERTISEMENT.efinition of PHENOLPHTHALEIN. : a white or yellowish-white crystalline compound C20H14O4 used in analysis as an indicator because its solution is brilliant red in alkalies and is decolorized by acids and in medicine as a laxative. See phenolphthalein defined for kids. ADVERTISEMENT.