Qualifications

It's shorter than a novella. It's not non-fiction. That pretty much covers it.

It's under 15,000 words long -- usually much under. 2000-5000 word stories will find the most markets these days.

Under 1000 words, and I call it a short-short. 0-250, I call flash fiction.

Other folks will have different word count standards.

Otherwise, it must be fictional. Can't say much more. Some short stories deal with a single incident in a single character's life. Others deal with entire lifetimes and the fates of many. Some have a tight, formal structure. Others are looser or experimental. Some are among the great works of literature. Others are purely ephemeral.

I'd have to say anything 10k and under. Stuff that's under 1k is flash. (Unless, of course, you're talking children's or YA fiction, in which case, the concept of a story vs. a book goes out the window.)

Like Phaeal mentioned, short stories vary a LOT in terms of what they cover, so drawing lines there doesn't work.

Keep in mind word count will vary from place to place. So you'll have to be aware of the guidelines for where you want to submit your piece.

Most places I've submitted want short fiction to be between 500-1,800 words. I end up naturally writing between 1,300-1,500. It's best to read the writer's guidelines for each publication first.

My idea of a short fiction story is that it must also have a beginning, middle, and end which flows well and leads to a satisfying conclusion. In my recent searches I seem to find a lot of short fiction doesn't seem to do that and I'm not sure if it means the format of short fiction has changed while I haven't been paying attention, or if it's just a whole different genre all together? I've just begun market research again after a few years hiatus, so things definitely seem different this go round.