The verb "illustrate" means to add pictures to a written work to clarify details written in the text or to explain an idea simply to aid understanding.
"Illustrate" comes from the Latin "illustratus," which means to make clear or to glorify an object or idea. The word entered the English language in the 16th century, when the word "illustration" could also be used to mean enlightenment or illumination, though these meanings are no longer used. Illustrated manuscripts have a long history in religious art, with the first usage of "illustrator" applied to a person who decorated books as early as the 12th century. Works produced in this fashion were brightly colored and often decorated with gold leaf.