I just did and I see nothing wrong with this derivative of an old word. I don't see why anyone would have a problem with understanding the meaning in context of the subject under discussion.
The "problem" is that it is a word you have made up that had no prior definition.
It was unclear if you meant
permissive, or
permitted, or some other known and understood word.
That you want it to be taken as the adjective of the noun
permittivity was not known, and, frankly, you've made rather a pig's dinner of trying to explain as much.
Actually, science hijacked the word and applied it to a very specific "condition" without asking the literary authority, if there is such a thing as electrical permittiveness.
???
Science didn't
hijack anything.
The word
permittivity did not exist prior to its
creation / coinage by Heaviside.
You can not hijack that which does not yet exist.
There is also no "literary authority", other than the population who use the language.
For a word to be coined it needs to be adopted.
Permittivity is such a word, with the definition given to it at the time.
Your
permittive, and
permittiveness are not.
At least yet.
That's not to say they won't be, if you keep pushing the word to all and sundry until it gets picked up and used.
But for now, your
permittive is everyone else's
"having non-zero permittivity" (or some such).
No one has granted science exclusive permission to claim the poorly chosen term "permittive" for a specific purpose to begin with. It actually interferes with the science of linguistics.
If you are confused by this, I can't help you.
It unfortunately seems that you are the one confused.
"Permittive" is not a word claimed by science.
Prior to you using it it was not recognised as a word in use at all, by anyone.
It is only claimed by
you for the meaning you have opted to give it.
"Permittivity", however, has been created by science for a particular purpose, as explained.
Does it interfere with the "since of linguistics"?
No.
It is an interesting sideshow that someone took the root word ("to permit") and came up with a new word for his purposes that alludes to the meaning of the root word.
But it does not interfere with linguistics.
Sure, we renamed it because....?
I don't know.
Maybe it was because the word
capacitance already existed and served the same purpose as Heaviside's
permittance?
And who gave him permission to invent (coin) a new word that really does not even represent the pertinent function?
Anyone can come up with a new word.
But one needs to clearly define it beforehand if it is to be understood by anyone coming across it.
It then needs to be widely adopted.
Good luck with this last part.
As for representing the pertinent
measure/property (not function),
permittivity can be understood as a measure of the opposition of the material to an electric field, so could be seen as how much the material
permits the electric field to behave, that sort of thing.
The term "permit" has nothing to do with electricity per se. It describes a degree of freedom of motion.
So what?
"Capacity" has nothing to do with electricity
per se.
"Resist" has nothing to do with electricity
per se.
"Impede", similarly, has nothing to do with electricity
per se.
Need I go on?
This does lend itself to why Heaviside opted for
permittivity rather than the more normal
permissivity, and that I do not know.
Maybe
permissive had other connotations in use that were less welcome.