A kind of latch whose bolt may be so locked by a detent that it can not be opened from the inside by the handle, or from the outside by the latch key.
deadlatch
night latch, night bolt, night lock
An auxiliary lock having a spring latch bolt which functions independently of the regular lock of the door; may be opened by a knob or handle from the inside but only by a key from the outside.ThesaurusSeenight latch