Origin: Latin suffix -ate
Illustrate has 4 different meanings across 1 category:
clarify by giving an example of
"The teacher used a diagram to illustrate how photosynthesis works."
depict with an illustration
"The textbook uses photographs to illustrate how plants grow in different climates."
To shed light upon.
"The teacher used diagrams to illustrate the complex scientific theory for her students."
The word illustrate is a back-formation created by removing the suffix -ation from illustration. It entered English as a verb form derived directly from the existing noun rather than originating independently.