\Phthal"ic\, a. [Naphthalene + -ic.] (Chem.)
Pertaining to, or designating, a dibasic acid obtained by the
oxidation of naphthalene and allied substances.
{Phthalic acid} (Chem.), a white crystalline substance,
   {C6H4.(CO2H)2}, analogous to benzoic acid, and employed in
   the brilliant dyestuffs called the phthaleins.