a white, solid substance, CH(CO)O, produced by the oxidation of naphthalene and used to make the phthalein dyes, certain synthetic resins, etc.
See phthalic anhydride in American Heritage Dictionary 4
noun
A white crystalline compound, C6H4(CO)2O, prepared by oxidizing naphthalene and used in the manufacture of phthaleins and other dyes, resins, plasticizers, and insecticides.