A visual representation (a picture or diagram) that is used to make some subject more pleasing or easier to understand
Open English WordNet
An illustration is a decoration, interpretation or visual explanation of a text, concept or process, designed for integration in print and digital published media, such as posters, flyers, magazines, books, teaching materials, animations, video games and films.
Picture, plate, diagram, plan, chart, map, design, or other graphic image printed with or inserted in the text of a book or other publication as an embellishment or to complement or elucidate the text
The visual representation of a person or an object.
A picture designed to decorate a volume or elucidate a literary work.