I write primarily fantasy; recently, it's been brought to my attention that I use the terms "muggle" and "squib" informally fairly often in roleplay. If I were to publish something, I'd not want to use Potterverse terms; "muggle" can be substituted with "mundane" or "normie", I suppose, though they don't have the same ring to them, but is there a better, less verse-specific term I can use instead of "squib" to mean "Someone who was expected to have magic due to magical heritage but who does not?"
Isn't one of the non-Rowling definitions of "squib" "a firecracker which doesn't explode"? So Rowling took something which means "has potential or is expected to do something, and fails to deliver," and used it for slang in a very appropriate way.
As I've said elsewhere, copy the work ethic, not the end result. Find or invent some other term which implies "fails to live up to potential or expectations" and use that instead.
You can cheat around it by just changing the words, but, really, you're still borrowing pretty heavily from someone else's universe just by using the concepts. Are they absolutely necessary? Is there some way you could come up with your own unique (or at least, somewhat less derivative) ideas of magic?
I'm not commenting on the legality or even the morality of borrowing the ideas, just the saleability. If you borrow a lot from Rowling, or from any other runaway success, you're just one of many opportunists trying to capitalize on her popularity. That means that you have to be better than a LOT of people, since there are a lot of people writing similar stuff. I think you'll have better luck at getting well-published if you set yourself apart at the concept stage, and THEN write a kick-ass story using your own set-up.