That's a great question, Jo_. And since no-one has asked it before, I've separated it out into a thread of its own.
In the Suddenly Forum, we define fiction no longer than 1,000 words as sudden fiction (also known as Flash Fiction). But that's a very generalized answer.
There are sub-sets and specific types of sudden fiction too. Drabble, for example, is exactly 100 words long.
And I'm going to shut up now because I want to hear other folks' interpretations of Sudden Fiction. Have at it, guys.