The Collaborative International Dictionary of English v.0.48:

Illumination \Il*lu`mi*na"tion\, n. [L. illuminatio: cf. F.
   illumination.]
   1. The act of illuminating, or supplying with light; the
      state of being illuminated.
      [1913 Webster]

   2. Festive decoration of houses or buildings with lights.
      [1913 Webster]

   3. Adornment of books and manuscripts with colored
      illustrations. See Illuminate, v. t., 3.
      [1913 Webster]

   4. That which is illuminated, as a house; also, an ornamented
      book or manuscript.
      [1913 Webster]

   5. That which illuminates or gives light; brightness;
      splendor; especially, intellectual light or knowledge.
      [1913 Webster]

            The illumination which a bright genius giveth to his
            work.                                 --Felton.
      [1913 Webster]

   6. (Theol.) The special communication of knowledge to the
      mind by God; inspiration.
      [1913 Webster]

            Hymns and psalms . . . are framed by meditation
            beforehand, or by prophetical illumination are
            inspired.                             --Hooker.
      [1913 Webster]

WordNet (r) 3.0 (2006):

illumination
    n 1: a condition of spiritual awareness; divine illumination;
         "follow God's light" [syn: light, illumination]
    2: the degree of visibility of your environment
    3: an interpretation that removes obstacles to understanding;
       "the professor's clarification helped her to understand the
       textbook" [syn: clarification, elucidation,
       illumination]
    4: the luminous flux incident on a unit area [syn:
       illuminance, illumination]
    5: painting or drawing included in a book (especially in
       illuminated medieval manuscripts) [syn: miniature,
       illumination]

Moby Thesaurus II by Grady Ward, 1.0:

187 Moby Thesaurus words for "illumination":
   abstract, abstraction, accomplishments, acquisition of knowledge,
   acquisitions, acrylic painting, adjunct, adornment, allegorization,
   altarpiece, arc lighting, arrangement, attainments, awareness,
   black and white, block print, brightness, catechization,
   chiaroscuro, clarification, coaching, collage, color,
   color patterns, color print, color printing, coloration, coloring,
   contrast, copy, cracking, cyclorama, daub, decipherment, decoding,
   decor, decoration, decorative lighting, demonstration,
   demythologization, didactics, diptych, direct lighting, direction,
   dyeing, edification, editing, education, elaboration,
   electric lighting, elucidation, emanation, embellishment,
   emblazonment, emblazonry, embroidery, emendation,
   encaustic cerography, encaustic painting, engraving, enlightenment,
   euhemerism, exegesis, exemplification, explanation, explication,
   exposition, expounding, festoon lighting, finger painting,
   floodlighting, flourish, flower arrangement, flower painting,
   fluorescence, fluorescent lighting, fresco, fresco painting,
   furniture arrangement, garnish, garnishment, garniture,
   gaslighting, genre painting, glow lighting, gouache, grisaille,
   guidance, highlight, highlights, icon, illustration, image,
   impasto, incandescence, incandescent lighting, indirect lighting,
   information, insight, instruction, irradiance, irradiancy,
   irradiation, learning, liberal education, light, light and shade,
   light source, lighting, likeness, lithography, luminosity,
   luminous energy, miniature, monochrome, montage, mosaic, mural,
   mural painting, oil painting, ornament, ornamentation,
   overhead lighting, painting, panorama, pedagogics, pedagogy,
   phosphorescence, photograph, photosensitivity, picture,
   picturization, pigmentation, portraiture, poster painting, print,
   private teaching, programmed instruction, radiance, radiancy,
   radiant energy, radiation, rationale, reason, reeducation,
   representation, reproduction, revelation, schooling,
   self-instruction, self-teaching, sidelight, simplification,
   solution, sophistication, spoon-feeding, spot lighting,
   stage lighting, stained glass window, staining, stencil,
   still life, store of knowledge, strip lighting, tableau, tapestry,
   teaching, the brush, tinction, tinging, tinting, tonality, trim,
   trimming, triptych, tuition, tutelage, tutorage, tutoring,
   tutorship, understanding, unlocking, visible radiation,
   wall painting, wash, wash drawing, window dressing