Source : Webster's Revised Unabridged Dictionary (1913)

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