Definitions of Flash Fiction
Other names for flash fiction include micro fiction, micro narrative, micro-story, postcard fiction, short short, short short story, and sudden fiction, though distinctions are sometimes drawn among some of these terms; for example, sometimes 1000 words is considered the cutoff between "flash fiction" and the slightly longer short story "sudden fiction". The terms "micro fiction" and "micro narrative" are sometimes defined as below 300 words. The term "short short story" was the most common term until about 2000, when it was overtaken by "flash fiction.�
Micro fictions are under 300. But as we have all seen lately, the six word stories are popular. I also just saw a call for a 58 word story. I guess our attention span has truly been whittled away.
Definition of Flash Nonfiction
All of the above but: memoir, essay, and factual writing in the very short form.