The Collaborative International Dictionary of English v.0.48:

Grate \Grate\, v. t. [imp. & p. p. Grated; p. pr. & vb. n.
   Grating.]
   To furnish with grates; to protect with a grating or
   crossbars; as, to grate a window.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Grating \Grat"ing\, n. [See 2d Grate.]
   1. A partition, covering, or frame of parallel or cross bars;
      a latticework resembling a window grate; as, the grating
      of a prison or convent.
      [1913 Webster]

   2. (Optics) A system of close equidistant parallel lines or
      bars, esp. lines ruled on a polished surface, used for
      producing spectra by diffraction; -- called also
      diffraction grating.

   Note: Gratings have been made with over 40,000 such lines to
         the inch, but those with a somewhat smaller number give
         the best definition. They are used, e. g., to produce
         monochromatic light for use in optical instruments such
         as spectrophotometers.
         [Webster 1913 Suppl.]

   3. pl. (Naut.) The strong wooden lattice used to cover a
      hatch, admitting light and air; also, a movable Lattice
      used for the flooring of boats.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Grating \Grat"ing\, a. [See Grate to rub harshy.]
   That grates; making a harsh sound; harsh. -- Grat"ing*ly,
   adv.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Grating \Grat"ing\, n.
   A harsh sound caused by attrition.
   [1913 Webster]

WordNet (r) 3.0 (2006):

grating
    adj 1: unpleasantly harsh or grating in sound; "a gravelly
           voice" [syn: grating, gravelly, rasping, raspy,
           rough, scratchy]
    n 1: a barrier that has parallel or crossed bars blocking a
         passage but admitting air [syn: grate, grating]
    2: a frame of iron bars to hold a fire [syn: grate, grating]
    3: optical device consisting of a surface with many parallel
       grooves in it; disperses a beam of light (or other
       electromagnetic radiation) into its wavelengths to produce
       its spectrum [syn: diffraction grating, grating]