What is an exposition?

The dictionary meaning of an expository writing is "a systematic interpretation or explanation (usually written) of a specific topic ". Another dictionary defines it as combination of "description, narration, or argument" . However, in my opinion, expository writing is an argumentative piece of writing which sips in some description and narration to explain the points systematically to interpret or explain a specific topic. My definition of exposition is a mixture of the two meanings from dictionaries.