Expository writing is

A piece of literature for the purpose of explaining.

Explanation:

Expository writing is writing that seeks to explain, illuminate or 'expose' (which is where the word 'expository' comes from). This type of writing can include essays, newspaper and magazine articles, instruction manuals, textbooks, encyclopedia articles and other forms of writing, so long as they seek to explain. Expository writing differs from other forms of writing, such as fiction and poetry.

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