It's maybe best described as a software ecosystem, which has the unfortunate problem that the programming language it uses and the package/deploy tools written in that programming language share the name "Nix". Nix the language (basically a customized version of Standard ML) is used to write package and OS definitions that Nix the tool uses to actually build and deploy software, containers, and operating systems. The "ecosystem" bit comes from the fact that the language is used to write the package/deploy definitions, and a large community of users have assembled a rather good and up-to-date set of package definitions, and these have been bundled together as an operating system, and these all share the rubric "Nix".