Flash Fiction Maven

A friend, Pam Casto, is undoubtedly the internet's
number one authority on Flash Fiction. If you have
the propensity for writing really short items, check
out her article and her forums. You may just feel
validated.

Early on in her article,

"Defining or stating exactly what flash fiction is would be comparable to defining or stating exactly what a poem or novel is. It just cannot be done to anyone's satisfaction. The main thing about flash fiction, however, is that it is short. Randall Jarrell once pointed out that a story can be as short as a sentence. And there are some fine flash fiction examples where stories, one or two pages long, are comprised of just one or two (sometimes quite long) sentences. But the number of sentences such work contains will not get us very far in defining or describing it."

http://www.geocities.com/kavitayan/flash01.html

This is her blog.

http://www.flashfictionblog.blogspot.com/