Let's just say nonfiction is a piece of
expository writing based in fact. Further definitions are as
follows: piece-a work with a beginning, a middle and an end.
Expository writing-writing with a purpose such as, but not limited
to, explanation, definition, information, description of a subject to
the extent that a reader will understand and feel something.  Please keep word count between 1,000 and 5,000 words.

Think
about the cave paintings of 30,000 years ago, they tell a story. And
for the modern man, a good film documentary conveys its purpose. A
film about Andy Warhol and his friends who liked to drink and smoke
and screw is interesting. A film about how I felt at age ten and
watching the adults in my life drink and smoke and screw is not a
good idea.

AND 

Base it in fact. Have at it. Choose something to write
about, learn about it and teach others. That's our definition of
nonfiction. We only demand that it's well written and that it serves
a purpose.

For a greater idea of what the nonfiction editor likes look here.

Please
double space. We do not accept multiple submissions, please wait for a
reply before submitting your next piece. Your work has to be previously
unpublished. We encourage you to submit your piece everywhere, but
please withdraw your piece if gets published elsewhere.

We use Submittable to accept and review our submissions.