trigger-n; 2 Senses

Sense Number 1: a lever that activates a firing mechanism, especially with a firearm

Commentary: TRIGGER[+artifact][+device][+lever][+activator][+of_mechanism][+firing]

There were not eyewitnesses who saw him pull the trigger of the gun.
When you release the trigger on this glue gun, it dispenses glue.

PropBank: NM
WordNet 3.0 Sense Numbers: 1, 2

Sense Number 2: an act that sets in motion a course of events

Commentary: TRIGGER[+role][+event][+catalyst][+for_course_of_events]

The assassination of the Archduke was the trigger that set of World War I.
The dry brush acted like a trigger for the forest fire.
The bank defaults were the initial trigger of the short-term credit crunch.

PropBank: trigger.01
WordNet 3.0 Sense Numbers: 3