To illustrate means to make something clearer by providing examples, pictures, or demonstrations. When your teacher illustrates a math concept by drawing a diagram on the board, she's helping you understand by showing you what it looks like. When a book about dinosaurs includes drawings of what scientists think T. rex looked like, those pictures illustrate the text.

If you're explaining why recycling matters, you might illustrate your point by describing how much trash your school produces in a week. Scientists illustrate their theories with experiments. A coach might illustrate a basketball play by moving players around to show where everyone should go.

An illustration is the example or picture itself. The colorful drawings in a picture book are illustrations. When someone asks you to illustrate your argument, they want you to give concrete examples, not just state your opinion. The word suggests making abstract ideas tangible and understandable, like turning “photosynthesis” from a complicated word into a clear diagram showing how plants use sunlight to make food.