To illustrate something is to explain it using examples, pictures, or stories so it becomes clearer. It can also mean to add drawings or images to a book or text.
From Latin "illustrāre" meaning "to light up, make clear, decorate," from "in-" (on) and "lustrāre" (to brighten). The idea of "making clear" naturally extended to explaining with pictures.
Illustrations hijack your visual brain, which is incredibly fast at spotting patterns—much faster than reading text. A single good diagram can compress pages of explanation into one instant "aha" moment.
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