Nixpkgs is certainly the core of the ecosystem, but it's not at all the whole ecosystem.
Other important parts of the NixOS ecosystem include:
- NixOS, the main module system which adds many configuration management capabilities to Nix
- Nix-Darwin and home-manager, extremely popular module systems for using Nix for configuration management on other operating systems
- various deployment tools: NixOps, Hail, Colmena, deploy-rs, Terranix
- pre-flake version pinning tools like Niv
- flake libraries like flake-utils, flake-utils-plus, std, and flake-parts
- development environment utilities and libraries like devshell, Lorri, and integrations like those of direnv and shadowenv
- the Nix User Repository and various important overlays (e.g., the Emacs overlay, the Rust overlays)
- the many 2nix conversion tools, some of which are chiefly distributed outside of Nixpkgs and used to package things in private or company collections rather than in Nixpkgs
- arguably some proprietary commercial products and SaaS offerings, like Cachix, Hercules CI, and Flox
- the NixOS hardware profiles collection
- the Nix and NixOS manuals
- important unofficial documentation sources, like nix.dev and nixos.wiki, and even important blog series like the Nix Pills or Ian Henry's reading of the official documentation
- wrappers and developer applications based on Nix, like devboxes and nixpacks
- GUI tools for working with Nix code or configuring NixOS, graphical app stores, etc.
- editor integrations like rnix-lsp, nix-mode, and environment management plugins for various editors
- a couple of really cool tools for automatically creating portable/deployable shell scripts with Nix (binlore + resholve)
- tools for generating magic fat binaries based on container technologies, like nix-bundle
- partial reimplementations of various parts of the Nix stack for specialized use in other programs, or sometimes aspiring to compete with Nix (like tvix)
Some of that stuff is included in Nixpkgs, but that's almost incidental— the ecosystem is not just the packages qua packages, but also various specialized tools and community knowledge which has been accumulated in the form of configuration modules and documentation. Those projects are first-class members of the Nix ecosystem in their own right, not only as things that might be included as packages in Nixpkgs :)
(And of course leveraging Nix successfully doesn't mean one has to evaluate or learn all of these tools. Most users will organically discover and stick with just a handful of them according to the demands of their own use cases. But in total, there is a really wide landscape of Nix-based and Nix-related software out there.)
In this case, "Nix" refers to the Nix CLI and the Nix language, and not to closely related projects like NixOS, Nixpkgs, Home Manager, and many others that can be understood as "ecosystem" projects dependent on the Nix CLI/language. It's also worth noting that there's a vast constellation of third-party Nix tools that can also be understood as dependent on the core NixOS/nix project on GitHub.
That's really just a formalization and documentation of the way things are named now. I don't think it precludes renaming some things in the long term.
Nix is the package manager that works on a multitude of operating systems. Then there was the thought “what if we use this same underlying technology to make a Linux distribution” and “we can manage services in a declarative manner” which became NixOS.
I thought Nix is the ecosystem?