Trigger

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.
Idioms
9.
quick on the trigger, Informal. quick to act or respond; impetuous; alert.
noun
1.
a small projecting lever that activates the firing mechanism of a firearm
2.
(machinery) a device that releases a spring-loaded mechanism or a similar arrangement
3.
any event that sets a course of action in motion
verb (transitive)
4.
(usually foll by off) to give rise (to); set off
5.
to fire or set in motion by or as by pulling a trigger
trick out

database
An action causing the automatic invocation of a procedure, for instance to preserve referential integrity. A triggers goes into effect when a user attempts to modify data with an insert, delete, or update command. A trigger can instruct the system to take any number of actions when a specified change is attempted. By preventing incorrect, unauthorized, or inconsistent changes to data, triggers help maintain the integrity of the database.
[Sybase SQL Server Release 10.0 Transact-SQL User’s Guide].
(1995-02-22)
In addition to the idiom beginning with trigger