Nix is a package builder & manager that creates reproducible build environments. Using the Nix language, you specify what the build environment should look like

Nix is a package builder & manager that creates reproducible build environments. Using the Nix language, you specify what the build environment should look like ($PATH, what should be in the build folder, etc). Nix then calls a program of your choice to do the actual building. Just like Nix lets you specify a build environment using the Nix language, NixOS lets you specify, using the Nix language, what you want your operating system to look like (systemd services, $PATH, etc). You should be able to use Nix, the package builder & manager, on any POSIX system afaik.