Nouns

(n)gun trigger, triggerlever that activates the firing mechanism of a gun
(n)triggera device that activates or releases or causes something to happen
(n)trigger, induction, initiationan act that sets in motion some course of events

Verbs

(v)trip, actuate, trigger, activate, set off, spark off, spark, trigger off, touch offput in motion or move to act“trigger a reaction”, “actuate the circuits”
(v)triggerrelease or pull the trigger on“Trigger a gun”