The Probert Encyclopaedia of Warfare

TRIGGER

A trigger is the portion of the small-arm mechanism actuated by the forefinger to fire the piece. In rifles and shotguns the trigger is a lever placed beneath the lock and usually protected by a small hoop of metal called the trigger guard. The lever of a hammer-fired weapon is in direct contact with the hammer when the piece is cocked, and releases the hammer upon pressure. In rifles the trigger actuates another lever called the sear which releases the striker when the trigger is pressed.
Research Trigger