[1] : The Collaborative International Dictionary of English v.0.48

radiation \ra`dia"tion\ r=a`di^=a"shu^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

[2] : WordNet (r) 2.0

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
     4: the spontaneous emission of a stream of particles or
        electromagnetic rays in nuclear decay syn: 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 radiation from a radioactive substance syn: radiotherapy,
         radiation therapy, actinotherapy, irradiation
see also:
radiation sickness radiation syndrome radioactivity radiotherapy radiation therapy actinotherapy 
irradiation 

[3] : 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

Radiation >> R Words
Radiation, definition of term: Radiation
radiation_pag1.html

Solar Radiation >> S Words
Solar Radiation, definition of term: Solar Radiation
solar+radiation_pag1.html

Gamma Radiation >> G Words
Gamma Radiation, definition of term: Gamma Radiation
gamma+radiation_pag1.html

Beta Radiation >> B Words
Beta Radiation, definition of term: Beta Radiation
beta+radiation_pag1.html

Alpha Radiation >> A Words
Alpha Radiation, definition of term: Alpha Radiation
alpha+radiation_pag1.html