Mainly the length. Flash fiction is up to 500 words (on this site), and short stories are up to 5000 words.
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To answer your question: It's all about word count. Really, that's all. Now, knowing what you can do with a limited word count is going to change your approach to the story, but that's a side effect, not a metric. A flash piece typically pushes a story forward in a way that explores an idea but it doesn't have the word count necessary to complete the journey. Because of that, flash often ends with the story still in motion. Not all the time, just often. Even a novel can do such a thing, but flash will do this just by its own nature. That quality really belongs to flash, and you need to embrace it if you really want say a lot in its brief pages (in its brief page, ha). You have a limited focus, and so you pour your efforts out tactically. I say this, but like all things, it's not true always. I wrote a flash piece here with 3 scenes once, just for fun.
<info hidden by Seven Crowns at poster's request> Thank you for that. Is there anyway for you as the mod to edit that part of my post that spills the beans, by chance?
This is actually a really important thing to remember. When you enter a competition, or submit a story for publication, you won't be able to revise it. You can't resubmit it and say, "oh sorry, I sent you the wrong version". Most publications will just ignore that and look at the original. And they won't accept a new submission later for a new version of a story they rejected. The same for competitions. You must make sure that you submit the version you're happy with. Sure, a lot of us do look back at submitted stories and think this or that bit could be better and want to revise it, but it's just a no-no. You'd can submit the revised story to a different place, not the same place. So always make sure, when you submit, the story is the version that you are happy with and can stand on its own two feet *as it is*.
Understood and agreed. However, my request <edited by Seven Crowns> is just about just deleting the part of my initial post up top.
There you go. I tucked away a couple posts, mine included, for mysterious reasons that the original posters understand. Especially with the forum moving on, not much is lost in this brief edit. I think that's enough to get us back to a safe place. Apologies to any toes I stepped on. Just pretend that nothing happened and let's let that stay fixed. . . . You always have to check with the publication too. Around here, 500 words is flash fiction. That's actually rather short. I feel 1000 words is more typical.