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]