My definition is pretty narrow and biased by what I've witnessed in the past elsewhere; its basically just hateful behavior, and maybe that's a better word for it. There's a sadistic factor where they enjoy and are entertained by harm coming to others, and a superiority factor where they hold themselves as better than those who emotionally react.
On the other hand, there's messing around with the mores and format of a space just in a spirit of fun and not hate - others might call that trolling, but I wouldn't use that word for it.
My bias is that I had a friend once who got into more hardcore trolling activity, and I ended up removing myself from them not because it affected me, but because I couldn't trust them knowing they enjoyed that. From very early on I felt there was something intensely traditional and normative to the unspoken philosophies of many who engaged in that despite the performance of being rebels - like, people who got targeted seemed always to be those who struggled with or openly flaunted the dominant cultural measures of success, age-appropriateness, manners, appearance and so forth.