The Collaborative International Dictionary of English v.0.48:

Radiant \Ra"di*ant\ (r[=a]"d[i^]*ant), a. [L. radians, -antis,
   p. pr. of radiare to emit rays or beams, fr. radius ray: cf.
   F. radiant. See Radius, Ray a divergent line.]
   1. Emitting or proceeding as from a center; resembling rays;
      radiating; radiate.
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   2. Especially, emitting or darting rays of light or heat;
      issuing in beams or rays; beaming with brightness;
      emitting a vivid light or splendor; as, the radiant sun.
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            Mark what radiant state she spreads.  --Milton.
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   3. Beaming with vivacity and happiness; as, a radiant face.
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   4. (Her.) Giving off rays; -- said of a bearing; as, the sun
      radiant; a crown radiant.
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   5. (Bot.) Having a raylike appearance, as the large marginal
      flowers of certain umbelliferous plants; -- said also of
      the cluster which has such marginal flowers.
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   6. (Physics) Emitted or transmitted by radiation; as, a
      radiant energy; radiant heat.
      [Webster 1913 Suppl.]

   Radiant energy (Physics), energy given out or transmitted
      by radiation, as in the case of light and radiant heat.

   Radiant heat, heat proceeding in right lines, or directly
      from the heated body, after the manner of light, in
      distinction from heat conducted or carried by intervening
      media.

   Radiant point. (Astron.) See Radiant, n., 3.
      [1913 Webster]

WordNet (r) 3.0 (2006):

radiant energy
    n 1: energy that is transmitted in the form of (electromagnetic)
         radiation; energy that exists in the absence of matter

Moby Thesaurus II by Grady Ward, 1.0:

31 Moby Thesaurus words for "radiant energy":
   contamination, curiage, decontamination, emanation, fallout,
   half-life, highlight, illumination, irradiance, irradiancy,
   irradiation, light, light source, luminous energy,
   natural radioactivity, nuclear radiation, photosensitivity,
   radiance, radiancy, radiation, radioactive radiation,
   radioactivity, radiocarbon dating, radiolucency, radiopacity,
   radiosensibility, radiosensitivity, saturation point, sidelight,
   specific activity, visible radiation