Source : Webster's Revised Unabridged Dictionary (1913)

Illustration \Il`lus*tra"tion\, n. [L. illustratio: cf. F.
   illustration.]
   1. The act of illustrating; the act of making clear and
      distinct; education; also, the state of being illustrated,
      or of being made clear and distinct.

   2. That which illustrates; a comparison or example intended
      to make clear or apprehensible, or to remove obscurity.

   3. A picture designed to decorate a volume or elucidate a
      literary work.

Source : WordNet®

illustration
     n 1: artwork that helps make something clear or attractive
     2: showing by example [syn: {exemplification}]
     3: an item of information that is representative of a type;
        "this patient provides a typical example of the syndrome";
        "there is an example on page 10" [syn: {example}, {instance},
         {representative}]
     4: a visual representation (a picture or diagram) that is used
        make some subject more pleasing or easier to understand