"Deadlatch" refers to locks that overcome the bypass technique often depicted in movies with a credit card.
Once properly engaged into the door frame, a deadlatch can't be pushed back in. In contrast, a 'nightlatch' or 'springlatch' is NOT a deadlatch, and IS susceptible to the old credit card trick.
'Deadlock' is another term entirely, which refers to the lock being key-lockable from the inside to prevent smashing an adjacent window and reaching in to open the door. A deadlatch may, or may not, be a deadlock.
"Deadbolts" effectively also have the deadlatching function, but require the bolt to be physically thrown by key or turnsnib, hence they come under a different category. A deadbolt may, or may not, be a deadlock.