The Collaborative International Dictionary of English v.0.48:

irradiation \ir*ra`di*a"tion\ ([i^]r*r[=a]`d[i^]*[=a]"sh[u^]n),
   n. [Cf. F. irradiation.]
   [1913 Webster]
   1. Act of irradiating, or state of being irradiated; as,
      irradiation of foods with X-rays can preserve their
      freshness by killing the bacteria that cause spoilage .
      [1913 Webster +PJC]

   2. Illumination; irradiance; brilliancy. --Sir W. Scott.
      [1913 Webster]

   3. Fig.: Mental light or illumination. --Sir M. Hale.
      [1913 Webster]

   4. (Opt.) The apparent enlargement of a bright object seen
      upon a dark ground, due to the fact that the portions of
      the retina around the image are stimulated by the intense
      light; as when a dark spot on a white ground appears
      smaller, or a white spot on a dark ground larger, than it
      really is, esp. when a little out of focus.
      [1913 Webster]

WordNet (r) 3.0 (2006):

irradiation
    n 1: the condition of being exposed to radiation
    2: a column of light (as from a beacon) [syn: beam, beam of
       light, light beam, ray, ray of light, shaft, shaft
       of light, irradiation]
    3: (physiology) the spread of sensory neural impulses in the
       cortex
    4: the apparent enlargement of a bright object when viewed
       against a dark background
    5: (Pavolvian conditioning) the elicitation of a conditioned
       response by stimulation similar but not identical to the
       original stimulus
    6: (medicine) the treatment of disease (especially cancer) by
       exposure to a radioactive substance [syn: radiotherapy,
       radiation therapy, radiation, actinotherapy,
       irradiation]

Moby Thesaurus II by Grady Ward, 1.0:

85 Moby Thesaurus words for "irradiation":
   X-ray dosimetry, X-ray therapy, anhydration, arc lighting,
   black and white, blast-freezing, bottling, brining, canning,
   chiaroscuro, cobalt therapy, contrast, corning, curietherapy,
   curing, decorative lighting, dehydration, desiccation,
   direct lighting, dry-curing, drying, electric lighting, emanation,
   embalming, enlightenment, evaporation, festoon lighting,
   floodlighting, fluorescent lighting, freeze-drying, freezing,
   fuming, gaslighting, glow lighting, highlight, highlights,
   illumination, incandescent lighting, indirect lighting,
   interstitial irradiation therapy, irradiance, irradiancy,
   irradiation therapy, isotope therapy, jerking, light,
   light and shade, light source, lighting, luminous energy,
   marination, mummification, overhead lighting, photosensitivity,
   pickling, potting, quick-freezing, radiance, radiancy,
   radiant energy, radiation, radiation therapy, radiotherapeutics,
   radiotherapy, radium therapy, radiumization, ray therapy,
   refrigeration, roentgen ray therapy, roentgenization,
   roentgenometry, roentgenotherapy, roentgentherapy, salting,
   seasoning, sidelight, smoking, spot lighting, stage lighting,
   strip lighting, stuffing, taxidermy, tinning, tonality,
   visible radiation