From all reference I have read, the phrase "exactly-once messaging" does not have a strict technical definition. Neither of your links define or use the exact term "exactly-once messaging". If you can point me to a definition of this specific phrase's standard terminology, I would be grateful. But, I cannot find one.

I don't understand why you address me with such a hostile tone. What have I done to deserve this? You accuse me of playing word games, but this is not my intent. I am just trying to have a conversation about the technical aspects of a system.