It may not be defined, but it can certainly be characterised. Umberto Eco has a good list: http://interglacial.com/pub/text/Umberto_Eco_-_Eternal_Fasci...

Missing from that page is the preamble in which he describes growing up under Mussolini.

> Purported "speech" that poses a clear and present danger is illegal.

Speech that leads to or implies violence at slightly greater remove is, however. Circulating pictures of targets with people's faces on; calling for violence or that people should arm themselves against an outgroup; that kind of thing.