Not even George Orwell, the great English Socialist, could answer that question definitively. Look up his essay "What is Fascism" (1944) where he laments the difficulty in pinning it down. Fascism, like pornography, may be hard to define but is easy to recognize when you see it.

Yet underneath all this mess there does lie a kind of buried meaning. To begin with, it is clear that there are very great differences, some of them easy to point out and not easy to explain away, between the régimes called Fascist and those called democratic. Secondly, if ‘Fascist’ means ‘in sympathy with Hitler’, some of the accusations I have listed above are obviously very much more justified than others. Thirdly, even the people who recklessly fling the word ‘Fascist’ in every direction attach at any rate an emotional significance to it. By ‘Fascism’ they mean, roughly speaking, something cruel, unscrupulous, arrogant, obscurantist, anti-liberal and anti-working-class. Except for the relatively small number of Fascist sympathizers, almost any English person would accept ‘bully’ as a synonym for ‘Fascist’. That is about as near to a definition as this much-abused word has come.

http://www.orwell.ru/library/articles/As_I_Please/english/efasc

Funny, I had to go to a Russian website to find the full text. But I guess Russians are OK, now.