The Collaborative International Dictionary of English v.0.48:

Phthalimide \Phthal"i*mide\, n. [Phthalic + imide.] (Chem.)
   An imido derivative of phthalic acid, obtained as a white
   crystalline substance, C6H4.(CO)2NH, which has itself (like
   succinimide) acid properties, and forms a series of salts.
   Cf. Imido acid, under Imido.
   [1913 Webster]