The Collaborative International Dictionary of English v.0.48:

Radiant \Ra"di*ant\, n.
   1. (Opt.) The luminous point or object from which light
      emanates; also, a body radiating light brightly.
      [1913 Webster]

   2. (Geom.) A straight line proceeding from a given point, or
      fixed pole, about which it is conceived to revolve.
      [1913 Webster]

   3. (Astron.) The point in the heavens at which the apparent
      paths of shooting stars meet, when traced backward, or
      whence they appear to radiate.
      [1913 Webster]

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.
      [1913 Webster]

   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.
      [1913 Webster]

            Mark what radiant state she spreads.  --Milton.
      [1913 Webster]

   3. Beaming with vivacity and happiness; as, a radiant face.
      [1913 Webster]

   4. (Her.) Giving off rays; -- said of a bearing; as, the sun
      radiant; a crown radiant.
      [1913 Webster]

   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.
      [1913 Webster]

   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
    adj 1: radiating or as if radiating light; "the beaming sun";
           "the effulgent daffodils"; "a radiant sunrise"; "a
           refulgent sunset" [syn: beaming, beamy, effulgent,
           radiant, refulgent]

Moby Thesaurus II by Grady Ward, 1.0:

227 Moby Thesaurus words for "radiant":
   ablaze, aerolite, aglow, alight, all-knowing, all-powerful,
   all-seeing, all-wise, almighty, arbor, axis, axle, axle bar,
   axle shaft, axle spindle, axle-tree, beaming, beamy, beatific,
   beatified, blazing, blessed, blissful, blithe, blithesome,
   blooming, blushing, bolide, boundless, bright, bright and sunny,
   brilliant, bubbling, bubbly, burning, candescent, capering,
   changeless, charismatic, cheerful, cheery, chirping, chondrite,
   coruscating, cosmic dust, creating, creative, dancing, dazzling,
   delighted, devastating, distaff, divine, ecstatic, effulgent,
   elated, eternal, eternally the same, eupeptic, euphoric,
   everlasting, exalted, exhilarated, exultant, fireball, flushed,
   flushed with joy, flushing, fulcrum, fulgent, gay, genial, gimbal,
   glad, gladsome, glamorous, gleaming, gleamy, gleeful, glinting,
   glistening, glittering, glorious, glossy, glowing, good, gorgeous,
   gudgeon, hallowed, happy, heavenly, high, highest, hinge, hingle,
   holy, hopeful, hub, illuminant, illustrious, immortal, immutable,
   in good spirits, in heaven, in high spirits, in seventh heaven,
   incandescent, infinite, irradiative, irrepressible, joyful, joyous,
   jubilant, just, killing, lambent, lamping, laughing, leaping,
   light as day, limitless, loving, lucent, luciferous, lucific,
   luciform, luminant, luminative, luminiferous, luminificent,
   luminous, lustrous, magic, majestic, making, mandrel, merciful,
   meteor, meteor crater, meteor dust, meteor shower, meteor swarm,
   meteor trail, meteor train, meteoric shower, meteorite, meteoroid,
   meteorolite, micrometeorite, micrometeoroid, nave, numinous,
   oarlock, of good cheer, omnipotent, omnipresent, omniscient,
   on cloud nine, one, optimistic, orient, overjoyed, permanent,
   perpetual, phosphorescent, pin, pintle, pivot, pleasant, pole,
   purring, radiant point, rapturous, raving, ravishing, refulgent,
   resplendent, rhapsodic, riant, rosy, rowlock, rutilant, rutilous,
   sacred, sanguine, sanguineous, scintillating, shaping, shimmering,
   shining, shiny, shooting star, siderite, siderolite, singing,
   smiling, smirking, sovereign, sparkling, spindle, splendent,
   splendid, splendorous, splendrous, starbright, starlike, starry,
   starry-eyed, streaming, stunning, sublime, suffused, sunny,
   sunshiny, supreme, swivel, tektite, thrice happy, timeless,
   trunnion, twinkling, ubiquitous, unbounded, unchanging, undefined,
   unlimited, winsome