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:

Diffraction \Dif*frac"tion\, n. [Cf. F. diffraction.] (Opt.)
   The deflection and decomposition of light in passing by the
   edges of opaque bodies or through narrow slits, causing the
   appearance of parallel bands or fringes of prismatic colors,
   as by the action of a grating of fine lines or bars.
   [1913 Webster]

         Remarked by Grimaldi (1665), and referred by him to a
         property of light which he called diffraction.
                                                  --Whewell.
   [1913 Webster]

   Diffraction grating. (Optics) See under Grating.

   Diffraction spectrum. (Optics) See under Spectrum.
      [1913 Webster]

WordNet (r) 3.0 (2006):

diffraction grating
    n 1: 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]