a white to pale-yellow, crystalline powder, CHO, used as a laxative, in making dyes, and as an acid-base indicator in chemical analysis: it is red in a solution containing a base and colorless in a solution containing an acid
See phenolphthalein in American Heritage Dictionary 4
A white or pale yellow crystalline powder, C20H14O4, used as an acid-base indicator, in making dyes, and formerly in medicine as a laxative. Because of its toxicity, it is no longer used in over-the-counter laxatives.