trigger

(noun) lever that activates the firing mechanism of a gun. Synonyms: gun trigger.

(noun) a device that activates or releases or causes something to happen.

(noun) an act that sets in motion some course of events. Synonyms: induction, initiation.

(verb) put in motion or move to act. Synonyms: trip, actuate, activate, set off, spark off, spark, trigger off, touch off. Examples: "Trigger a reaction." "Actuate the circuits."

(verb) release or pull the trigger on. Example: "Trigger a gun."