lever that activates the firing mechanism of a gun
"The hunter carefully pulled back on the trigger to release the arrow."
a device that activates or releases or causes something to happen
"The safety mechanism acts as a trigger that releases the hammer when pulled back."
an act that sets in motion some course of events
"The sudden announcement acted as a trigger for a series of unexpected protests across the city."
A finger-operated lever used to fire a gun.
"He reached out and pulled the trigger of his revolver."
In plain English: A trigger is something that causes an event to happen immediately, like pulling the lever on a gun to fire it.
"The loud noise acted as a trigger for his anxiety attack."
Usage: In everyday language, this noun refers specifically to the mechanical part of a firearm that releases ammunition when pulled by the trigger finger. Avoid using it as a synonym for an abstract cause or catalyst unless you are quoting idiomatic expressions like "triggering off" a reaction.
To fire a weapon.
"The soldier pulled the trigger to fire his rifle at the enemy."
The word originally came to English as "tricker" from the Dutch trekker, which referred to a pulling device like a drawer handle or bell pull. It entered our language via the verb trekken, meaning to drag, draw, or pull.