A 5 lever mortice lock is a standard type of lock that deadlocks, (unlike a yale lock that operates on a catch).
The word mortice refers to how the lock is fixed to the door. Instead of being mounted on the surface of the door, the lock is inserted into the door itself, in a special hole called a mortice that is normally used in wooden joints. Being as the lock is held within the door’s structure, rather than screwed on, much more force is required to break in through the door and so it is considered more secure.
Having 5 levers in the lock mechanism makes the lock more secure as well. Each lever has to be moved correctly for the bolt to be released, so it is more difficult to pick a lock with 5 rather than 3 levers because of the added complexity.
Deadlocking means the lock’s bolt can be moved back and forth to open and close the door using the key only, rather than other locks which may also feature a lever and catch to accomplish this without the necessity of having the key actually present.
This picture shows a typical mortice lock mounted beneath the door handle. Often the faceplate of the lock will have a British Standard (BS) kite mark and the number of levers it has, stamped upon it in the vicinity of the red X marking.
Some insurers will want confirmation that the locks on a property meet a certain standard, by specifying a BS kitemark number, or how many levers the lock should have, to meet the level of security required to effect cover on their policy.