trig·ger

[trig-er]

–noun
1. a small projecting tongue in a firearm that, when pressed by the finger, actuates the mechanism that discharges the weapon.

2.a device, as a lever, the pulling or pressing of which releases a detent or spring.

3.anything, as an act or event, that serves as a stimulus and initiates or precipitates a reaction or series of reactions.

4.Slang . triggerman

–verb (used with object)
5.to initiate or precipitate (a chain of events, scientific reaction, psychological process, etc.): Their small protest triggered a mass demonstration.

6.to fire or explode (a gun, missile, etc.) by pulling a trigger  or releasing a triggering device: He accidentally triggered his rifle.

–verb (used without object)
7.to release a trigger.

8.to become active; activate.

—Idiom
9. quick on the trigger, Informal . quick to act or respond; impetuous; alert.