The Collaborative International Dictionary of English v.0.48:

Radiation \Ra`di*a"tion\ (r[=a]`d[i^]*[=a]"sh[u^]n), n. [L.
   radiatio: cf. F. radiation.]
   1. The act of radiating, or the state of being radiated;
      emission and diffusion of rays of light; beamy brightness.
      [1913 Webster]

   2. The shooting forth of anything from a point or surface,
      like the diverging rays of light; as, the radiation of
      heat.
      [1913 Webster]

WordNet (r) 3.0 (2006):

radiation
    n 1: energy that is radiated or transmitted in the form of rays
         or waves or particles
    2: the act of spreading outward from a central source
    3: syndrome resulting from exposure to ionizing radiation (e.g.,
       exposure to radioactive chemicals or to nuclear explosions);
       low doses cause diarrhea and nausea and vomiting and
       sometimes loss of hair; greater exposure can cause sterility
       and cataracts and some forms of cancer and other diseases;
       severe exposure can cause death within hours; "he was
       suffering from radiation" [syn: radiation sickness,
       radiation syndrome, radiation]
    4: the spontaneous emission of a stream of particles or
       electromagnetic rays in nuclear decay [syn: radiation,
       radioactivity]
    5: the spread of a group of organisms into new habitats
    6: a radial arrangement of nerve fibers connecting different
       parts of the brain
    7: (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:

111 Moby Thesaurus words for "radiation":
   X ray, actinic ray, actinism, arc lighting, asteroids, atomic beam,
   atomic ray, attenuation, aurora particles, beam, beam of light,
   black and white, blackout, broadcast, broadcasting, chiaroscuro,
   circumfusion, contrast, cosmic particles, cosmic ray bombardment,
   decorative lighting, diffraction, diffusion, dilution,
   direct lighting, dispensation, dispersal, dispersion,
   dissemination, dissipation, distribution, divergence,
   electric lighting, emanation, emission, enlightenment, evaporation,
   expansion, festoon lighting, floodlighting, fluorescent lighting,
   fragmentation, gamma ray, gaslighting, gleam, glow lighting,
   highlight, highlights, illumination, incandescent lighting,
   indirect lighting, infrared ray, intergalactic matter,
   invisible radiation, irradiance, irradiancy, irradiation, leam,
   light, light and shade, light source, lighting, luminous energy,
   meteor dust impacts, meteors, overhead lighting, patch, pencil,
   peppering, photon, photosensitivity, propagation, publication,
   radiance, radiancy, radiant energy, radiorays, radius, ray,
   ray of light, ribbon, ribbon of light, scattering, scatterment,
   shedding, shotgun pattern, sidelight, solar rays, sowing,
   space bullets, spattering, splay, spoke, spot lighting, spread,
   spreading, sprinkling, stage lighting, streak, stream,
   stream of light, streamer, strewing, strip lighting, the bends,
   tonality, ultraviolet ray, violet ray, visible radiation,
   volatilization, weightlessness