What is an exposition?

From Novelguide.com:

Exposition: Writing intended to explain the nature of an idea, thing, or theme. Expository writing is often combined with description, narration, or argument. In dramatic writing, the exposition is the introductory material which presents the characters, setting, and tone of the play.

From Literary Terms:

EXPOSITION
A writing or a speech that explains a process or presents information. In
plot structure, the exposition is the part of
the work that introduces the characters, the setting,
and the basic situation.

From AskOxford:

exposition

• noun 1 a comprehensive description and explanation of a theory. 2 a large public exhibition of art or trade goods. 3 Music the part of a movement in which the principal themes are first presented.

— DERIVATIVES expositional adjective.

— ORIGIN Latin, from exponere ‘present, explain’.

In terms of music (Wikipedia):

In musical form and analysis, exposition is the initial presentation of the thematic material of a musical composition, movement, or section. The use of the term generally implies that the material will be developed or varied.

The briefest definition I've found (Butte College):

Exposition: Writing that explains.

I'm not sure what else to put... :(

Edit (23/3):
Just got the materials today. The handout states that "an exposition is a rhetorical discourse that provides information about or an explanation of difficult material." Seems like a clearer definition to me, compared to what I referenced above...