It's got elements of all of those things. It's basically a toolchain for building an 'environment', which is roughly analogous to a Linux distribution.
It starts with a language that lets you declare the desired state of your environment, including which packages are present and the configuration of those packages. The packages are installed and managed through the Nix package manager. The end result is an 'environment' that reflects the desired state you expressed. That environment can be a Docker image, an ISO, or it could be a running system you're booted into (in the case of NixOS). Or it can even be an ephemeral environment that exists on the filesystem of whatever distribution you're using (in the case of nix-shell). Each of these options offers different levels of isolation and reproducibility, depending on the requirements of your project or system.
There's lots of clever components that make something like this possible, and they're all wrapped up in the Nix umbrella.
It starts with a language that lets you declare the desired state of your environment, including which packages are present and the configuration of those packages. The packages are installed and managed through the Nix package manager. The end result is an 'environment' that reflects the desired state you expressed. That environment can be a Docker image, an ISO, or it could be a running system you're booted into (in the case of NixOS). Or it can even be an ephemeral environment that exists on the filesystem of whatever distribution you're using (in the case of nix-shell). Each of these options offers different levels of isolation and reproducibility, depending on the requirements of your project or system.
There's lots of clever components that make something like this possible, and they're all wrapped up in the Nix umbrella.