Extremism may still breach civil or criminal laws, including for hate speech. But it would not trigger more serious terrorism offences - or else we would be in the realm of violent extremism.
How do we decide?
By calling something or someone extremist, we are saying the beliefs and behaviours fall outside legitimate contributions to the public sphere. This must mean something more than views we consider to be highly controversial, offensive or generally unpalatable.
Agreeing on this thin dividing line is probably an impossible task. Extremism occupies a liminal space where someone's beliefs and behaviours do not involve or advocate violence, but there is a strong public sense that they are, nonetheless, undermining the terms of our (increasingly fragile) social contract.
Still, we should not be afraid to call out extremist behaviour where we see it. But we should take into account what the term means and consider whether the behaviour fits the definition.
Then, extremism will not just be another buzzword, but a term with real meaning that can help us all to determine appropriate limits for democratic debate.