What do you call this?
I think it's a kind of lock, but don't know what exactly its name is.
Ayuda-Tulong, thanks for your quick reply.
It's called a "dead-bolt lock"?
Certainly the lock doesn't seem to depend on a bolt,
though I'm not an expert on etymology...
Oh, it's the bolt. I see.
But why it is called a 'dead-bolt lock' still remains unsolved.
I have heard it referred to as just a 'dead lock' in the UK.
a deadlock as Tdol says, sometimes a deadbolt, or a mortice lock. I have never heard dead-bolt lock,
'Dead lock' means you need a key to lock it and unlock it, so it differs from a 'latch' that is spring loaded and locks when you shut the door, without needing a key.
'Mortice lock' means you have to cut a hole, called a mortice (same as a mortice and tenon joint), in your door for the lock to go in.