Source : Webster's Revised Unabridged Dictionary (1913)

Apple pie \Ap"ple pie`\
   A pie made of apples (usually sliced or stewed) with spice
   and sugar.

   {Apple-pie bed}, a bed in which, as a joke, the sheets are so
      doubled (like the cover of an apple turnover) as to
      prevent any one from getting at his length between them.
      --Halliwell --Conybeare.

   {Apple-pie order}, perfect order or arrangement. [Colloq.]
      --Halliwell.

Source : WordNet®

apple pie
     n : pie (with a top crust) containing sliced apples and sugar