Indeed all references to that I am aware of talk about it at the “nixOS module system”, rather than “nix module system”.
This is where my confusion stems from.
I agree that coining a new term that finds its way into official glossaries and doesn’t imply dependency to NixOS would be a good thing.
Though “nix modules” might be sound to much as if there was some native support for them in nix itself, while “nixpkgs modules” again would feel weird, as nixpkgs itself doesn’t use them at all (but in fact provides them as part of its “stdlib”).