Jared Paquette had a post that referred to The New York Review of Books article Ur-Fascism by Umberto Eco, June 22, 1995 Issue.
But in spite of this fuzziness, I think it is possible to outline a list of features that are typical of what I would like to call Ur-Fascism, or Eternal Fascism. These features cannot be organized into a system; many of them contradict each other, and are also typical of other kinds of despotism or fanaticism. But it is enough that one of them be present to allow fascism to coagulate around it.
The article goes on to describe 14 features. What I like about the article is that it seems to explain why there are so many different descriptions of what fascism is.
The original article Jared pointed to is Umberto Eco Makes a List of the 14 Common Features of Fascism. This article adds some more modern examples to the 14 features than Eco could have written about in the original 1995 article. For someone of my age, it is hard to think of a 1995 article as being almost prehistoric.