Source : Webster's Revised Unabridged Dictionary (1913)

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.

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

   {Diffraction grating}. (Optics) See under {Grating}.

   {Diffraction spectrum}. (Optics) See under {Spectrum}.

Source : WordNet®

diffraction
     n : when light passes sharp edges or goes through narrow slits
         the rays are deflected and produce fringes of light and
         dark bands