Definitions for "Deadlatch"
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.
A nightlatch or latch, the springbolt of which can be locked (or deadlocked) by key or other means.
A latch in which the latchbolt is positively held in the projected position by an auxiliary mechanism.