I told someone I just had a piece of flash fiction published in a local magazine and he seemed puzzled. "Flash fiction?" I offered, "Short-short story? Sudden fiction? Mine is 449 words; flash is usually under 500 words." He still hadn't heard of it. I said if you go to nanofiction.org you can find hundreds of examples, and those are all under 300 words. It is possible that the term originated from the 1992 book, Flash Fiction: 72 Very Short Stories, an anthology of stories under 750 words, those that fit on a two-page spread.
While I do not think it can be defined and boxed to everyone's satisfaction, flash does have some recurring characteristics. It packs details and action and story and character into one distilled package. Sometimes it's a meditation. Something large or very small can happen. It often has a twist or a turn at the end, perfect for bookmaking. The language can be rich, almost chewy. Lydia Davis is an author I admire who writes very short pieces that could be called flash.
Whatever it is or isn't, here my story "Tati's Necklace" in its online incarnation. If you live in the East Bay and an actual paper copy appears on your doorstep, you can find it on page 11 of the December issue of The Monthly.