The quick and inadequate definition of nonfiction
is that it is NOT fiction.
By the way, nonfiction is the correct spelling.
There is no English language word non, so it is incorrect
to write the term in hyphenated format. In other words, there
is no English non fiction or non-fiction, only English
nonfiction.
The prefix non means an absence. So when we say something
is nonfiction, we are saying there is no fiction in it.
Fiction is prose that is imagined, invented, fabricated,
or feigned.
2 types of writing: prose & poetry
Prose, of course, is language that’s primarily concerned
about the ideas conveyed. It has little interest in how the ideas
sound.
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Poetry, by contrast, cares about the music of the words
as much as the ideas.
I know this is boring, but you need to get these definitions
down cold. The fiction/nonfiction distinction is a basic literary
concept, roughly on the level of the 3-times tables.
If your students can't write a paragraph answer to the question
"what is nonfiction?" they shouldnt get a high
school diploma.
Imaginative writing genres
Let’s look at genres (types or classes) of the imaginative or
fictive side of writing to get a feel for what nonfiction isn’t.

You can see that short stories and novels are fictional
genres. Most plays are prose fiction, but some (like
Shakespeare's plays) are in poetry form.
Novels, plays, and short stories employ setting, characterization,
and dialogue to relate a series of related events we call a
story.
As you can see from the diagram, it is absurd to talk about a
nonfiction novel. A novel is fiction and there is
no fiction in nonfiction.
That language is less apt to create confusion than saying "nonfiction
story." Students understand what true means, but nonfiction
is a pretty hazy concept for most of them. Of course, some students
will misunderstand no matter what you say. Be prepared to teach
the fiction-nonfiction distinction many times!
The study of English language and literature is full of such
terminology traps for the unwary student and his/her teacher.
To see what nonfiction is,
instead of what it is not, we’ll need a fresh page and another
diagram.