Flash fiction is a term used for short-short stories. I picked up a book recently called The World's Shortest Stories, which is a collection of short stories just 55 words long, based on the 55 Fiction Concept. 55 words doesn't seem like a lot until you compare it to those who have limited themselves to even shorter lengths, such as the aforementioned 6 word stories.
Here's an example of the concept applied to non-fiction, courtesy of Pyromaniacs. 50 words is a severe limit no matter what you're writing, but particularly in the area of non-fiction. Kudos to the Pyro guys for coming up with posts that are both entertaining and edifying.
If that's not concise enough for you, Abraham Piper tries keeps his posts to 22 words. All I can say is wow, wow, wow, wow, wow. Verbal decluttering, indeed.
For the rest of use mere mortals, 800 words is perhaps a more reasonable cutoff (suggested by Andrée Seu in a reprint of a World Magazine article on writing. (HT: JT).