Do you already know one?
I think ER's right that there's "NO" definition--I think he means no "official" one. And not really any definitive all-encompassing one. Umberto Eco wrote up a list of 14 characteristics of fascism, and argued that it took only one for a society to qualify as fascist. I think that might be a bit extreme, but in the article I first read, the author argued that Trump is demonstrating 7 of the 14. Hell yeah he is.
What's your definition of fascism, Rob? Do you think Donald Trump is a fascist (or his election would lead to fascism)?
(If it helps any, I recognize that Trump isn't pushing conservative policies, more a butcher's counter of fresh meat he's dangling in front of his mob. His message is populist fascism, not conservatism.)