Designed to explain or inform; serving to set forth facts, ideas, or information clearly and directly.
From Latin exponere (to set forth) + -ory (suffix meaning 'of or relating to'). This became standard English in the 16th century for describing explanatory writing or speech.
Your English class probably teaches 'expository writing' as distinct from narrative or persuasive writing—it's the writing mode where your job is pure explanation, like instruction manuals or encyclopedia entries.
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