Word that describes a word which isn't normally used in an everyday conversation

Solution 1:

It's possible you just want

esoteric

adjective

intended for or likely to be understood by only a small number of people with a specialized knowledge or interest. [Google D]

Solution 2:

I'm guessing rare, obscure, and arcane are not rare, obscure, or arcane enough for you, so how about

recondite: little known; abstruse.

or

recherché: rare, exotic, or obscure.

Which definition has the precise example, in the dictionary, of:

"a few linguistic terms are perhaps a bit recherché for the average readership"

If you want to say a rare word belongs to a vocabulary used only in technical contexts or by specific subcultures, you could say it is jargon, lingo, or argot.

Coming back to the need for a noun, you could always say:

My, that is a lyrical locution