Nix is a package manager, and nixpkgs is its "official package respositry." Both of those pieces should work on any (decently popular) POSIX system. NixOS is a

Nix is a package manager, and nixpkgs is its "official package respositry." Both of those pieces should work on any (decently popular) POSIX system. NixOS is a Linux distribution that uses Nix in place of apt or yum or what-have-you, and uses nix to allow the entire system to be configured declaatively.