The Collaborative International Dictionary of English v.0.48:

Pistachio \Pis*ta"chio\, n. [It. pistacchio (cf. Sp. pistacho,
   F. pistache), fr. L. pistacium, Gr. ?, ?, fr. Per. pistah.
   Cf. Fistinut.] (Bot.)
   The nut of the Pistacia vera, a tree of the order
   Anacardiace[ae], containing a kernel of a pale greenish
   color, which has a pleasant taste, resembling that of the
   almond, and yields an oil of agreeable taste and odor; --
   called also pistachio nut. It is wholesome and nutritive.
   The tree grows in Arabia, Persia, Syria, and Sicily. [Written
   also pistachia.]
   [1913 Webster]

   2. The small anacardiaceous tree (Pistacia vera), of
      southern Europe and Asia Minor, which bears the pistachio
      nut. Called also pistache.
      [Webster 1913 Suppl.]

   3. (Cookery) The flavor of the pistachio nut, or an ice or
      confection flavored with it.
      [Webster 1913 Suppl.]

   4. Pistachio green.
      [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

pistachio nut
    n 1: nut of Mediterranean trees having an edible green kernel
         [syn: pistachio, pistachio nut]