The more I think about it, the more it seems like a mistake to nail these terms down precisely. If we try to, then whenever a comment doesn't match the definition, people will argue that it must be ok because it doesn't technically break the guidelines. That's not what we want to encourage. We want HN to be motivated by the spirit of the law more more than its letter.
It's true that "troll" is ill-defined. Perhaps we can try not to use that word. I like the phrase "toxic comments" because, while imprecise, it conveys what those do to the ecosystem. In practice, I'm not sure this is as big a problem as it seems. As Sam pointed out, the bulk of the community has little trouble recognizing these things.
I think some of the lessons from "Raising a Moral Child" from two days ago fit here. We tell people "don't be a jerk or troll", but when an individual makes a toxic comment, we tell them "you're a good person, even if you wrote a toxic comment, and we know you can do better".
I like "toxic" too.