Common sense is the collection of what's been learned about nature and people and by the age of 18.
It's not as quotable as Einstein but is more broadly applicable.
There is no need to define hostility. Any adult has, by definition, lived through childhood and puberty. Children are naturally cruel. Teens have mastered socialization skills well enough to elevate cruelty to a team sport.
I'm not saying every child is cruel or that every teen is a member of a group that uses cruelty as an instrument of power. But we've all seen it. Many of us have been personally guilty of cruelty towards others at least once or twice. It probably left a bad taste in one's mouth - bad enough to recognize that's not the person you want to be. But I doubt any of us is a virgin in this matter.
Moderation is done by a team for a reason. A team has a leader for a reason. If a team member strays, other members help nudge that person back on the correct path. If the behavior continues, the team leader gets involved in the nudging. If the behavior persists, the team member is dropped.
That sends an important message to the remaining team members. One, their efforts are supported by leadership. Two, no one person is so critical to the success of the team that they can't be dropped for failing to comport themselves according to the mission and values of the team.
There's no need for a definition of what hostility is. We know it when we see it...because we've seen it, been the target of it, or the deployer of it.
Personally, I believe the line is crossed when a member addresses their criticism at a person as opposed to the argument. When that line is crossed - when the negativity gets personal - that's when moderation is needed.
Of course, that's not the only manner in which a person can intentionally disrupt a discussion. The person who repeatedly & persistently gets involved in discussions to stir the pot and sew the seeds of frustration amongst others - trolling - doesn't insult others and may not even actually engage in the discussion other than to disagree.
The troll doesn't argue from a position of logic. They're disagreement isn't founded on a rational understanding of a fact or concept. Nor is their participation triggered by a recognition that someone has misrepresnted a fact or or misunderstood a concept.
The troll simply disagrees. They contribute gibberish - nothing of value - to the discussion. They ignore rational commentary made in response to their gibberish. They persistently disagree simply for the pleasure of watching others agonize over their irrationality, and refusal to acknowledge facts or logic.
The troll's objective is to be a metasticized cancer in a thread. They've succeeded when the majority of content added to the thread is in response to their gibberish. These posts do nothing to further an understanding of the central topic. The thread now exists solely to feed the engorged ego of the troll. All attention is focused on them and their (arguably benign) antagonism.
We have members who are trolls in this forum. They've been active from day one and have worked hard to transform the site into a stagnant swamp. They can be recognized by their thread-derailing comments that end up in the dumpster.
I just hope the dumpster area isn't too little, too late.
No, we don't need a definition of hostility. We need a membership and moderation team that's empowered to nudge a person back in line when their criticisms are directed at people. We need a leadership team that will not tolerate trolling. We also need a leadership team that will nudge a moderator back in line when they step in where moderation isn't needed.