I think you're describing a https://en.wikipedia.org/wiki/Dedekind_cut https://en.wikipedia.org/wiki/Dedekind_cut (edit: oh, I see the article makes that clear

I think you're describing a https://en.wikipedia.org/wiki/Dedekind_cut https://en.wikipedia.org/wiki/Dedekind_cut (edit: oh, I see the article makes that clear. Well here's a link anyway)