Mike Snider's Formal Blog and Sonnetarium: "Poetry just isn't a kind of fiction, though some poems do imitate the form of actions which no one, including the author, believes to have actually occurred."

That's a pretty good definition of fiction. We don't assume that Shelley really thought he could talk to the West Wind. Whether the author him or herself would have used that exact word is another question. I believe Sophocles is a writer of fiction, even though he believes the characters he is writing about actually existed. He imitates an action that he doesn't believe occured in exactly that way. There is nothing in the concept of fiction (from Latin fingere) that makes it applicable only to narrative prose.

Aside from that, though, I agree with pretty much everything in Mike Snider's post.