The Collaborative International Dictionary of English v.0.48:

Milling \Mill"ing\, n.
   The act or employment of grinding or passing through a mill;
   the process of fulling; the process of making a raised or
   intented edge upon coin, etc.; the process of dressing
   surfaces of various shapes with rotary cutters. See Mill.
   [1913 Webster]

   High milling, milling in which grain is reduced to flour by
      a succession of crackings, or of slight and partial
      crushings, alternately with sifting and sorting the
      product.

   Low milling, milling in which the reduction is effected in
      a single crushing or grinding.

   Milling cutter, a fluted, sharp-edged rotary cutter for
      dressing surfaces, as of metal, of various shapes.

   Milling machine, a machine tool for dressing surfaces by
      rotary cutters.

   Milling tool, a roller with indented edge or surface, for
      producing like indentations in metal by rolling pressure,
      as in turning; a knurling tool; a milling cutter.
      [1913 Webster]

WordNet (r) 3.0 (2006):

milling machine
    n 1: machine tool in which metal that is secured to a carriage
         is fed against rotating cutters that shape it [syn:
         miller, milling machine]