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
    n 1: when light passes sharp edges or goes through narrow slits
         the rays are deflected and produce fringes of light and
         dark bands

Moby Thesaurus II by Grady Ward, 1.0:

76 Moby Thesaurus words for "diffraction":
   amplitude, antinode, attenuation, broadcast, broadcasting,
   circumfusion, crest, de Broglie wave, deflection, deflexure,
   diffusion, dilution, dispensation, dispersal, dispersion,
   dissemination, dissipation, distortion, distribution, divergence,
   electromagnetic radiation, electromagnetic wave, evaporation,
   expansion, flection, flexure, fragmentation, frequency,
   frequency band, frequency spectrum, guided wave, in phase,
   interference, light, longitudinal wave, mechanical wave, node,
   out of phase, peppering, period, periodic wave, propagation,
   publication, radiation, radio wave, ray, refraction, reinforcement,
   resonance, resonance frequency, scatter, scattering, scatterment,
   seismic wave, shock wave, shotgun pattern, skewness, sound wave,
   sowing, spattering, splay, spread, spreading, sprinkling, strewing,
   surface wave, tidal wave, torsion, transverse wave, trough,
   volatilization, wave, wave equation, wave motion, wave number,
   wavelength