trigger

Concise Oxford English Dictionary © 2008 Oxford University Press:
trigger/ˈtrɪɡə(r)/

▶noun

• 1 a device that releases a spring or catch and so sets off a mechanism, especially in order to fire a gun.

• 2 an event that causes something to happen.

▶verb

• 1 cause (a device) to function.

• 2 (also trigger something off) cause to happen or exist.

– derivatives
triggered adjective.

– origin C17: from dial. tricker, from Du. trekker, from trekken ‘to pull’.