You're using a different definition to suit your argument. I've stayed with mine this entire time. You can't debate two separate definitions for existence then use them interchangeably. That's ludicrous. 

By definition, it did not exist. Existence did not exist prior to defining existence. This is a philosophical matter more so than a scientific one since we are using a particular definition here.

"Existence did not exist prior to defining existence."
As a term I certainly agree, but as a rule in the universe? I'd like a source for that.

Knowing that we are here, requiring matter to cause us to exist, matter necessarily needed to exist. At the moment that no sentient being were alive, matter needed to exist following the rules of causality, whether we were there to observe it or not.

The application of the definition is predicated upon a third party source, that's why I mentioned the mirror effect. It's no different from the chicken and the egg, really. 

Again, you're defining existence entirely different than I am. In my defintion, nothing existed until it was observed. You have to play by the rules. If you want to define existence in another manner, do so. But if you do not and you continue to misappropriate my definition, we won't get anywhere.

Then the predicate of your definition is inadquate, given the exception case I offered. At a moment when no 3rd party source existed, and we know by causality that matter needed to exist prior to the existence of said 3rd party source, the predicate is inadequate in answering the existential questions of life.

There are no exceptions, it's a definition. Conclusions can only be drawn from the definition. By definition, if it had not been observed it did not exist. This is a very simple concept to grasp. 

Example. Confining the Universe to a solid box. There may or may not be a ball inside of the box. I don't know, I haven't yet observed it. Nor has anyone else. It does not exist. I open the box, ALAS! a ball! It does exist. Understand? This ties into quantum theory, if you're aware. Schrodinger's cat experiment. 

Okay, I can rewire my brain to understand existence as what you defined it as. It is not the word I use when I say "nothing disproves that God exists", it is not the same word as the one you use. The word I use does not imply that "no observation = no (?)existence(?)".

-> My question then would be, what is the term used to say it's there but we haven't observed it yet?

I've heard of Shrodinger's cat experiment, and read about it a while ago, do you think I should read more about it to understand better?

It's interesting, but it's probably beyond this discussion. [Schrodinger's cat experiment]

Yeah, you do have to rewire your brain when you confine to a particular definition. 

You're using the same term just definining it differently than I am which is fine, but you need to define it first so we can use your definition instead if you want to go that route. But don't expect to debate my definition using yours. That doesn't work.

Because we both know that matter "existed" prior to the generation of sentient beings, I personally feel in safer logic using the definition I'm supporting.

So far, I have gathered that existence is a property independent of the observer, but how is that property defined? Self-defined? "I exist"?

For the example of the inception of sentient beings, by causality.

A is required for B to exist, therefor prior to B's existence, A existed.

It would be defined as "A exists" or "A existed at time t".