Wikipedia
NixOS
NixOS is a Linux distribution that aims to improve the state of the art in system configuration management. Built on top of the Nix package manager, it uses declarative configuration and allows reliable system upgrades. Two main branches are offered: current Stable release and Unstable following latest development.
Although NixOS started as a research project, it is a fully functional and usable operating system.
NixOS is DevOps friendly and has tools dedicated to deployment tasks.