I think of a "trigger" as something that initiates/sets off a reaction. In the context of mental health issues, I think of it as something that sets off symptoms. This can be an urge to engage in a behavior or thoughts and feelings.

PTSD triggers are one type of trigger, but that usage doesn't get a monopoly on the word that's been around since long before PTSD was recognized. If people mean PTSD trigger then they can say PTSD trigger. (this annoyance isn't directed at you. But there are people who argue that it should only be used for PTSD triggers and that people using it for other mental illness symptoms is akin to people who use bipolar to mean moody or saying they're "so ADHD" when they mean they're distracted or whatever, and I very strongly disagree)

Trigger is used outside of the context of mental illness all the time. My interpretation wrt mental illness is that it doesn't have a formal definition. If something triggers my urge to SH or triggers intense anxiety or whatever other million symptoms I have then I'm going to call it a trigger ¯\_(ツ)_/¯