noun

1. a small lever or part which when pulled or pressed releases a catch, spring, etc.
2. in firearms, a small lever pressed back by the finger to activate the firing mechanism
3. an act, impulse, etc. that initiates an action, series of events, etc.

Etymology: earlier tricker < Du trekker < trekken, to draw, pull: see trek

transitive verb

1. to fire or activate by pulling or pressing a trigger
2. to initiate (an action); set off the fight that triggered the riot