What to do if you don't know the terms you don't know? If in a (mathematics) research paper, you cannot seem to find the term to describe something even after much digging and questioning, is it safe to assume that a word for it does not exist? 

This question becomes even more confounding the more simple the concept. For example, I cannot seem to find a common term describing the first index in a sequence that a number divides (the index it is a primitive factor of). 

If this assumption is not a safe one, how should I proceed? With one of the few things stalling the self-archiving of a paper being the ambiguity over a term, do I simply assume that there is not a common term and create my own at the risk of seeming like an ignoramus? Is there any other course of action that may be taken, any general approach that other researches have taken?
It's a common problem, not only in mathematics but in all fields of research. Oftentimes, we feel that there should be a term to describe a specific concept or method, but we sure don't know what it is or how to find it if it exists. Scouring the literature for a term we don't know is very difficult, if not impossible, as long as we don't have at least a direction to look for.

Basically, the best thing to do in that case is to do due diligence (e.g., ask colleagues in a suitable field, look over papers that you suspect may have used this concept or method, check some text books), but if that ends up being unfruitful you can either coin your own term or not use a specific term at all. For the former, an academic's favorite cop-out phrase ("to the best of our knowledge ...") may come in handy.

The good news is that, if an established term actually exists, you may very easily be pointed towards it during peer review.