A topic, phrase or word that emotionally sets someone off. Could refer to anger, or reliving a traumatic experience. Sometimes this is logical, other times it seems like an attention grab. It is fairly common for certain communities on, for example, Livejournal to be overly sensitive to people's 'triggers' (often but not always relating to sexual assault)--any mention of violence may be censored.
(1) Man, I know I shouldn't have gone off on Eric that way, but that racist joke he made triggered me big time and I flipped my shit.
(2) I got kicked off that livejournal group for not lj-cutting my post talking about the rape scene in the movie I watched last night. Some girl said it triggered her and the mods were on her side.