The Collaborative International Dictionary of English v.0.48:

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.
      [1913 Webster]

   2. That which illustrates; a comparison or example intended
      to make clear or apprehensible, or to remove obscurity.
      [1913 Webster]

   3. A picture designed to decorate a volume or elucidate a
      literary work.
      [1913 Webster]

WordNet (r) 3.0 (2006):

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