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

Moby Thesaurus II by Grady Ward, 1.0:

148 Moby Thesaurus words for "illustration":
   abstract, abstraction, acrylic painting, allegorization, alphabet,
   altarpiece, art, block print, blueprint, case, case history,
   case in point, charactering, characterization, chart, choreography,
   citation, clarification, collage, color print, coloring,
   conventional representation, copy, cracking, cross reference,
   cyclorama, dance notation, daub, decipherment, decoding,
   delineation, demonstration, demythologization, depiction,
   depictment, diagram, diptych, drama, drawing, editing, elucidation,
   emblem, emendation, encaustic cerography, encaustic painting,
   engraving, enlightenment, euhemerism, example, exegesis, exemplar,
   exemplification, explanation, explication, exponent, exposition,
   expounding, figuration, figure, finger painting, flower painting,
   for instance, fresco, fresco painting, genre painting, gouache,
   grisaille, hieroglyphic, icon, iconography, ideogram, illumination,
   image, imagery, imaging, impasto, instance, item, letter, light,
   likeness, limning, logogram, logograph, map, miniature, monochrome,
   montage, mosaic, mural, mural painting, musical notation, notation,
   object lesson, oil painting, painting, panorama, particular,
   photograph, pictogram, picture, picturization, plan, portraiture,
   portrayal, poster painting, prefigurement, presentment, print,
   printing, problem, projection, quotation, rationale, realization,
   reason, reference, relevant instance, rendering, rendition,
   representation, representative, reproduction, sample, sampling,
   schema, score, script, simplification, solution, specimen,
   stained glass window, stencil, still life, syllabary, symbol,
   tablature, tableau, tapestry, the brush, triptych, type,
   typical example, unlocking, wall painting, wash, wash drawing,
   writing