The Collaborative International Dictionary of English v.0.48:
Phthalic \Phthal"ic\, a. [Naphthalene + -ic.] (Chem.) Pertaining to, or designating, a dibasic acid obtained by the oxidation of naphthalene and allied substances. [1913 Webster] Phthalic acid (Chem.), a white crystalline substance, C6H4.(CO2H)2, analogous to benzoic acid, and employed in the brilliant dyestuffs called the phthaleins. [1913 Webster]