The adjective "well-illustrated" is a modern English compound formed by combining the intensifying adverb "well" with the past participle "illustrated." The core verb "illustrate" originates from the Latin illustrare, meaning "to light up, illuminate, make clear," which itself is derived from in- (upon) and lustrare (to illuminate, purify). The metaphorical shift from physical lighting to intellectual clarification is ancient, as Roman writers used the term for rhetorical elucidation. By the 17th century, "illustrate" in English specifically acquired the sense of providing explanatory pictures, leading to "well-illustrated" as a descriptor for works where visual examples thoroughly clarify the subject matter.

Structure: well (thoroughly) + illustrat (depict) + ed (adjective suffix) - well: From Old English wel, meaning "in a good way, thoroughly." It functions as an intensifying adverb in this compound. - illustrat: A direct substring of the English verb illustrate, from Latin illustratus, the past participle of illustrare meaning "to light up, make clear, explain." It serves as the core verbal root. - ed: From Old English -ed, -od, a suffix forming past participles and adjectives from verbs, indicating the state of having undergone the action.