exposition

US /ˌek.spəˈzɪʃ.ən/
UK /ˌek.spəˈzɪʃ.ən/
"exposition" picture
1.

a comprehensive description and explanation of an idea or theory

:
The book provides a clear exposition of the author's philosophy.
His lecture was a brilliant exposition of quantum mechanics.
2.

a large public exhibition of art or trade goods

:
The city is hosting an international trade exposition next month.
We visited the art exposition at the new gallery.
3.

the part of a play or work of fiction that provides the background information necessary to understand the plot

:
The first act of the play serves as the exposition, introducing the main characters and setting.
A good novel often has a subtle exposition woven into the early chapters.