Want to see how people define it.
Most Helpful Girl
It is something liberals came up with because they can't handle other people's opinions.6
Most Helpful Guy
I would give a legitimate definition as being speech that is directly hostile, prejudiced and discriminatory with no room for ambiguity. It is speech that encorages hatred and division. It is speech that could lead to violence, if not directly advocating it. It is not anything that might offend someone, it is not subjective to any degree, it is objectively hateful.
This is not an exhaustive list and there would have to be a lot more clarifications and clauses to make it watertight, but it gives a feeling of where my opinion lies.2