The Collaborative International Dictionary of English v.0.48:

Flour \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.
   [1913 Webster]

   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 Flour 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.
      [1913 Webster]

WordNet (r) 3.0 (2006):

flour mill
    n 1: a mill for grinding grain into flour