NixOS is a Linux distribution built around the Nix package manager. The entire system, including the kernel, applications and configuration files, is described declaratively in the Nix language, usually in a single configuration file. A command evaluates that definition and builds a new system generation without overwriting earlier versions. Upgrades are transactional, and an unwanted update can be rolled back to a previous generation. Software comes from the Nixpkgs collection. NixOS is installed on your own hardware or virtual machines and is aimed at developers, administrators and organisations that want reproducible systems.
NixOS is open source under the MIT licence. The project is supported by the NixOS Foundation, a Dutch non-profit registered with the Chamber of Commerce in Utrecht. Among other things, the Foundation runs the binary cache cache.nixos.org and the build server hydra.nixos.org. Because the operating system is fully open and runs on systems you control, a European organisation can manage and audit its configuration and build chain itself, without depending on a single supplier.