Simply speaking, a dead lock (also known as deadbolt) is a more secure form of lock than a Yale lock or other spring-loaded lock. The lock does not rely on a spring to unlock it, and it will not lock automatically when you shut the door, as a Yale lock will do.
“Deadlock: A lock having only a square-ended deadbolt operable from one or both sides by key, and occasionally from outside only by key, inside by thumb turn. Sometimes operable only from outside and with no inside keyhole, which is designated a single-entry deadlock.”
Master Locksmith Association
You can find more information here: http://www.locksmiths.co.uk/wp-content/uploads/2012/04/MLA-Lock-related-Jargon-Buster.pdf
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