Source : Webster's Revised Unabridged Dictionary (1913)

Irradiation \Ir*ra`di*a"tion\, n. [Cf. F. irradiation.]
   1. Act of irradiating, or state of being irradiated.

   2. Illumination; irradiance; brilliancy. --Sir W. Scott.

   3. Fig.: Mental light or illumination. --Sir M. Hale.

   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.

Source : WordNet®

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}]
     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 radiation from a radioactive substance [syn: {radiotherapy},
         {radiation therapy}, {radiation}, {actinotherapy}]