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]

WordNet (r) 3.0 (2006):

phthalic acid
    n 1: a colorless acid used to make dyes and perfumes