The Collaborative International Dictionary of English v.0.48:

Illuminate \Il*lu"mi*nate\, v. t. [imp. & p. p. Illuminated;
   p. pr. & vb. n. Illuminating.] [L. illuminatus, p. p. of
   illuminare; pref. il- in + luminare to enlighten, fr. lumen
   light. See Luminous, and cf. Illume, Illumine,
   Enlimn, Limn.]
   1. To make light; to throw light on; to supply with light,
      literally or figuratively; to brighten.
      [1913 Webster]

   2. To light up; to decorate with artificial lights, as a
      building or city, in token of rejoicing or respect.
      [1913 Webster]

   3. To adorn, as a book or page with borders, initial letters,
      or miniature pictures in colors and gold, as was done in
      manuscripts of the Middle Ages.
      [1913 Webster]

   4. To make plain or clear; to dispel the obscurity to by
      knowledge or reason; to explain; to elucidate; as, to
      illuminate a text, a problem, or a duty.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Illuminating \Il*lu"mi*na`ting\, a.
   Giving or producing light; used for illumination.
   [1913 Webster]

   Illuminating gas. See Gas, n., 2
   (a) .
       [1913 Webster]

WordNet (r) 3.0 (2006):

illuminating
    adj 1: tending to increase knowledge or dissipate ignorance; "an
           enlightening glimpse of government in action"; "an
           illuminating lecture" [syn: enlightening,
           informative, illuminating] [ant: unenlightening,
           unilluminating]

Moby Thesaurus II by Grady Ward, 1.0:

52 Moby Thesaurus words for "illuminating":
   allegorizing, annotative, autodidactic, brightening, clarifying,
   coeducational, critical, cultural, demonstrative, demythologizing,
   didactic, disciplinary, edifying, editorial, educating,
   educational, educative, elucidative, enlightening, euhemeristic,
   exegetic, exemplificative, exhortatory, explaining, explanatory,
   explicative, explicatory, expositive, expository, glossarial,
   homiletic, hortatory, illuminant, illuminative, illumining,
   illustrative, informative, initiatory, instructive, introductory,
   lecturing, lightening, lighting, preaching, preceptive,
   propaedeutic, rationalistic, rationalizing, scholiastic,
   self-teaching, teaching, tuitionary