il·lus·trate / ˈiləˌstrāt/ • v. [tr.] provide (a book, newspaper, etc.) with pictures: the guide is illustrated with full-color photographs. ∎  explain or make (something) clear by using examples, charts, pictures, etc.: the results are illustrated in Figure 7. ∎  serve as an example of: a collection of pieces that illustrate Bach's techniques.ORIGIN: early 16th cent. (in the sense ‘illuminate, shed light on’): from Latin illustrat- ‘lit up,’ from the verb illustrare, from in- ‘upon’ + lustrare ‘illuminate.’

il·lus·tra·tion / ˌiləˈstrāshən/ • n. a picture illustrating a book, newspaper, etc.: an illustration of a yacht. ∎  an example serving to clarify or prove something: this accident is a graphic illustration of the disaster that's waiting to happen. ∎  the action or fact of illustrating something, either pictorially or by exemplification: by way of illustration, I refer to the following case.DERIVATIVES: il·lus·tra·tion·al / -shənl/ adj.

illustrate throw light or lustre on; elucidate XVI; exemplify; elucidate with pictures XVII. f. pp. stem of L. illustrāre, f. IL-1 + lustrāre illuminate, rel. to lūmen LIGHT1.
So illustration †illumination XIV; exemplification, example XVI; pictorial elucidation XIX. — (O)F. — L. illustrative XVII. illustrious distinguished by rank, etc. XVI. f. L. illustris.