expositional

noun

1. a setting forth of facts, ideas, etc.; detailed explanation
2. writing or speaking that sets forth or explains
3. an exhibition; esp., a large public exhibition or show, often international in scope
4. that part of a play, etc. which reveals what has happened before, who the characters are, etc.
5. the first section of certain musical forms, which introduces the main theme or themes, as in a sonata, or all the voices, as in a fugue