trigger

ot noun

1. A finger-operated lever used to fire a gun. Just pull the trigger.
2. An event that initiates others, or incites a response. Sleeping in an unfamiliar room can be a trigger for sleepwalking.
3. A concept or image that upsets somebody. I can't watch that violent film. Blood is one of my triggers.
4. A similar device used to activate any mechanism.

fe’l verb

1. To initiate something. The controversial article triggered a deluge of angry letters from readers.
2. To spark a response, especially a negative emotional response, in (someone). This story contains a rape scene and may be triggering for rape victims.
3. To fire a weapon.
4. To activate; to become active.