The Collaborative International Dictionary of English v.0.48:

Phenolphthalein \Phe`nol*phthal"e*in\, or Phenol phthalein
\Phenol phthalein\, n. (Chem.)
   A white or yellowish white crystalline substance, C20H14O4,
   formed by condensation of the anhydride of phthalic acid and
   phenol. Its solution in alkalies is brilliant red, but is
   decolorized by acids. This reaction, being very delicate, is
   used as a pH indicator. It is sometimes used as a laxative in
   veterinary medicine.
   [Webster 1913 Suppl. +PJC]