This is not an easy question. Yes - a short story is a work of fiction almost always. A non-fiction short narrative that utilizes literary devices would be called "creative non-fiction" these days.
The "single effect" definition offered in post 6 seems more cut-and-dry than anything I can come up with, but I would write my one sentence definition like this:
A short story is a work of fiction seeking to express an idea, describe a person or place, or convey a feeling through prose and can be distinguished from a novel by a narrowness of scope and usually presents its narrative in a single, linear time frame.