The Collaborative International Dictionary of English v.0.48:

Illustrative \Il*lus"tra*tive\, a.
   1. Tending or designed to illustrate, exemplify, or
      elucidate.
      [1913 Webster]

   2. Making illustrious. [Obs.]
      [1913 Webster]

WordNet (r) 3.0 (2006):

illustrative
    adj 1: clarifying by use of examples [syn: exemplifying,
           illustrative]
    2: serving to demonstrate [syn: demonstrative, illustrative]