Flash fiction, short shorts, or short-short fiction are all what they sound like: small, complete fiction stories told as briefly as possible. Length varies depending on whom you're talking to and what contest you're entering. The Southeast Review dubs it as any piece of fiction 500 words or shorter for their World's Best Short Short Story Contest (WBSSSC). Glimmer Train states for their Very Short Fiction Award that it's anything less than 3,000 words but they note that they don't think anyone can write a good story in less than 500. Not that I'm trying to start a Lit Mag cat fight.
Ahem. Many of my short shorts start off as poems that has awkward childhoods. They knew they weren't really supposed to be poems but they were born to parents that were poems and in between poem children and had to grow up in order to find out their place in the world was among fiction, though they'll always love their foster family. (Was that last little tid bit in the Ugly Ducking story? I can't recall. Probably just my modern therapy adding on information.)