Exposition, n.
Etymology: [L. expositio, fr. exponere, expositum: cf. F. exposition. See Expound.]
1. The act of exposing or laying open; a setting out or displaying to public view.
2. The act of expounding or of laying open the sense or meaning of an author, or a passage; explanation; interpretation; the sense put upon a passage; a law, or the like, by an interpreter; hence, a work containing explanations or interpretations; a commentary. You know the law; your exposition Hath been most sound. Shak.
3. Situation or position with reference to direction of view or accessibility to influence of sun, wind, etc.; exposure; as, an easterly exposition; an exposition to the sun. [Obs.] Arbuthnot.
4. A public exhibition or show, as of industrial and artistic productions; as, the Paris Exposition of 1878. [A Gallicism]
Shades of meaning
Fernald has no entry of its own for exposition; he lists it under definition.
Where he writes about it:
An exposition undertakes to state more fully what is compactly given or only implied in the text; as, an exposition of Scripture. — from definition
Definition, explanation, exposition, and interpretation are ordinarily blended in a commentary, which may also include description. — from definition