trigger

A trigger is the mechanism on a gun that makes it shoot a bullet or anything that starts a series of events. (noun)

An example of trigger is what a cop presses to fire his gun.
An example of trigger is a hostile confrontation that causes a series of bad memories with the person.

Trigger means to fire a gun or to start a series of events or emotions. (verb)

An example of trigger is to shoot a rifle while hunting.
An example of trigger is to start crying at a movie, thereby causing many other people in the theater to begin weeping.

See trigger in Webster's New World College Dictionary