The word hermeticity as (for the lack of better definition, hence the question) “the quality of being hermetic” (not to be confused with mathematical hermiticity, which is also absent from the general dictionaries) seems to be widely used in technical literature. Yet, I am having hard time finding a bona fide definition in any decent dictionary or encyclopædia available on-line. Britannica acknowledges its existence, and via OneLook I only got one reference to the translation from German Hermetizität. OED, Cambridge and M-W turn up nothing.
It could be the case that the use of the suffix -ity to form “nouns denoting quality or condition” is sufficient, but why, for instance, have plasticity if I can get the definition “the quality of being easily shaped or moulded” from the definition of plastic: “(of substances or materials) easily shaped or moulded” (all quotes from OED)? Why not the same treatment of hermeticity?
It can be presumed that such word might appear in unabridged dictionary, but then again, why this term (in all forms, such as hermetic) is not more widely used? From my experience people prefer using the terms water- or air-tight, but hermetic works better both as a general term, and in situations when saying “air-tight liquid hydrogen container” is as semantically wrong as it is technically non-descriptive (it may be air– or water-tight, but not liquid hydrogen-tight).
I think, the usage frequency angle is important in this question. It seems strange to me that this term (again, in all forms) is not widely understood (or used) amongst English-speakers (again, from my limited experience), while, for example, any graduate of a Russian public grade school who took a science class (i.e. any) would understand this borrowed word, even though there are native equivalents for air– and water-tight.
Best Answer
It's the same reason dictionaries don't usually explicitly list standard plurals, verb inflections, etc.
In the case of plasticity, for example, the base word plastic has multiple "shades" of meaning. Without the definition, you might think perhaps it meant the proportion of plastic incorporated in a composite material. Okay, not likely, but you get my drift.
In the case of hermetic, it doesn't really matter if you don't know which precise meaning is embodied in hermeticity (so far as I'm aware, effectively they all are). Occam's razor and all that.
It's not all that common, and anyone reading it who doesn't recognise the word couldn't fail to discover an entry for hermetic if they check any dictionary. They'll figure it out.
It's also worth copying in this from OED
That's one of about a dozen definitions for hermetic in the OED entry, and all the rest relate closely to the original literal pertaining to Hermes Trismegistus, and the ... writings ascribed to him.
From which I conclude that standalone adjectival hermetic in OP's sense is probably relatively new, and the extrapolation from this to an abstract noun for quality of being hermetic even newer. Perhaps the dictionaries just haven't caught up yet.