a diagram, chart, or picture that accompanies a written work in order to make it more attractive or clarify some part of it;
e.g. The book had an illustration of the dragon in the story.
(noun)
an example or demonstration that serves to prove something or make it clearer;
e.g. The misunderstanding provided an illustration of why we need better communication.
(noun)
the action of showing something and making some aspect of it clearer, usually by providing examples or visual media, or the state of being thus shown/clarified;
e.g. She used several graphs for the illustration of her argument.