Wikipedia
A dead bolt, deadbolt or dead lock is a locking mechanism distinct from a spring bolt lock because a deadbolt cannot be moved to the open position except by rotating the lock cylinder with the key. The more common spring bolt lock uses a spring to hold the bolt in place, allowing retraction by applying force to the bolt itself. A deadbolt can therefore make a door more resistant to entry without the correct key.
A deadbolt is often used to complement a spring-bolt lock on an entry door to a building.
Usage examples of "dead bolt".
Easing it shut, I slid the oiled dead bolt in place behind me, feeling better despite knowing most of the I.
As he waited for the elevator, he heard the dead bolt slide into place as she locked the office doors.
Unlocking the dead bolt, she slid off the chain and threw open the door.