Source : Webster's Revised Unabridged Dictionary (1913)

Flour \Flour\, n. [F. fleur de farine the flower (i.e., the
   best) of meal, cf. Sp. flor de la harina superfine flour,
   Icel. fl["u]r flower, flour. See {Flower}.]
   The finely ground meal of wheat, or of any other grain;
   especially, the finer part of meal separated by bolting;
   hence, the fine and soft powder of any substance; as, flour
   of emery; flour of mustard.

   {Flour bolt}, in milling, a gauze-covered, revolving,
      cylindrical frame or reel, for sifting the flour from the
      refuse contained in the meal yielded by the stones.

   {Flour box} a tin box for scattering flour; a dredging box.
      

   {Flour} {dredge or dredger}, a flour box.

   {Flour dresser}, a mashine for sorting and distributing flour
      according to grades of fineness.

   {Flour mill}, a mill for grinding and sifting flour.

Source : WordNet®

flour mill
     n : a mill for grinding grain into flour