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]