The Collaborative International Dictionary of English v.0.48:

Phthalide \Phthal"ide\, n. [Phthalyl + anhydride.] (Chem.)
   A lactone (C8H6O2) obtained as a white crystalline
   substance by reduction of phthalyl chloride and other
   processes; hence, by extension, any one of the series of
   which phthalide proper is the type. [Written also
   phthalid.]
   [1913 Webster]

   Note:
         [PJC]

   2. Specifically: A fungicide (C8H2Cl4O2) which is a
      tetrachloro derivative of phthalide[1] proper; in this
      sense, the spelling fthalide is also used. It has CAS
      Registry Number 27355-22-2.

   Syn: Syn. 4,5,6,7-tetrachloro-1(3H)-isobenzofuranone; -- .
        [PJC]