Thanks Peter. That's really interesting. I'd always used - and heard - the term "dielectric constant" though understood that "permittivity" is the same thing. The link in the Wikipedia article is broken so I looked it up. Ultimate ends up here:

https://ieeexplore.ieee.org/document/8657413/definitions

As if to make the point, "dielectric constant" is listed but not defined. "Permittivity", however, is extensively defined. Not sure it's accessible if you're not an IEEE member.

Mike