The Collaborative International Dictionary of English v.0.48:

Illustrate \Il*lus"trate\, v. t. [imp. & p. p. Illustrated; p.
   pr. & vb. n. Illustrating.] [L. illustratus, p. p. of
   illustrare to illustrate, fr. illustris bright. See
   Illustrious.]
   1. To make clear, bright, or luminous.
      [1913 Webster]

            Here, when the moon illustrates all the sky.
                                                  --Chapman.
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   2. To set in a clear light; to exhibit distinctly or
      conspicuously. --Shak.
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            To prove him, and illustrate his high worth.
                                                  --Milton.
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   3. To make clear, intelligible, or apprehensible; to
      elucidate, explain, or exemplify, as by means of figures,
      comparisons, and examples.
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   4. To adorn with pictures, as a book or a subject; to
      elucidate with pictures, as a history or a romance.
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   5. To give renown or honor to; to make illustrious; to
      glorify. [Obs.]
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            Matter to me of glory, whom their hate
            Illustrates.                          --Milton.
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The Collaborative International Dictionary of English v.0.48:

Illustrate \Il*lus"trate\, a. [L. illustratus, p. p.]
   Illustrated; distinguished; illustrious. [Obs.]
   [1913 Webster]

         This most gallant, illustrate, and learned gentleman.
                                                  --Shak.
   [1913 Webster]

WordNet (r) 3.0 (2006):

illustrate
    v 1: clarify by giving an example of [syn: exemplify,
         illustrate, instance]
    2: depict with an illustration
    3: supply with illustrations; "illustrate a book with drawings"

Moby Thesaurus II by Grady Ward, 1.0:

102 Moby Thesaurus words for "illustrate":
   account for, adorn, adumbrate, allegorize, approve, argue, attest,
   bespeak, betoken, body forth, breathe, call to mind, cite,
   cite a particular, clarify, clear up, connote, crack, decipher,
   decorate, demonstrate, demythologize, denote, display, document,
   elucidate, embellish, emblazon, embody, enlighten, euhemerize,
   evidence, evince, example, exemplify, exhibit, explain,
   explain away, explicate, exposit, expound, express, figure,
   foreshadow, furnish evidence, give a for-instance,
   give indication of, give reason for, give the meaning, go to show,
   illuminate, image, impersonate, imply, incarnate, indicate,
   instance, involve, itemize, make clear, make plain, manifest, mark,
   mirror, name, ornament, particularize, personate, personify,
   picture, point to, popularize, prefigure, pretypify, project,
   quote, rationalize, realize, reflect, set forth, shadow,
   shadow forth, shed light upon, show, show how, show signs of,
   show the way, signalize, signify, simplify, solve,
   speak for itself, speak volumes, spell out, suggest, symptomatize,
   tell, tend to show, throw light upon, unfold, unlock, unravel