The Nix ecosystem is a software ecosystem that has the Nix package manager and language at its core but also includes a vast network of related tools and systems built on top of Nix. On the official side, the Nix ecosystem includes NixOS, NixOps, Hydra, Home Manager, and other projects. On the unofficial side, the Nix ecosystem includes everything from language libraries and helpers to package repositories to developer tools and far beyond.
Depending on your perspective, Nix-inspired projects like GNU Guix and projects that share Nix's commitment to principles like reproducibility and declarative configuration may also fall under the Nix ecosystem umbrella.
Regardless of where you draw the lines, getting into Nix can bring vast regions of the software industry into your reach.