Two simple definitions of writing

Fiction and Literature (any genre)
Using primarily words, the rearrangement of reality to reveal and explore the human condition.

Non-Fiction (any genre)
Using primarily words, the notation of aspects of reality to reveal and explore the human condition.

Usually the definition is something like this but this doesn't really mean much to a writer:

Writing

Noun:

The activity or skill of marking coherent words on paper and composing text.  The activity or occupation of composing text for publication: "she made a decent living from writing".