Nix

While not immediately apparent, the name Nix refers to a number of different tools and concepts:

Nix (package manager): tooling for managing packages and build processes using the Nix language;
Nix (language): a declarative language used for defining expressions that are evaluated by the various Nix tools;
Nix (command): the new experimental CLI interface that aims to unify the various Nix commands;
Nix (command line utilities): a set of CLI tools that evaluate Nix expressions with the goal of producing various builds;
Nix store: a place that can store file system data (such as packages), in particular, the Nix store found at /nix/store;
The Nix ecosystem, a set of tools and programs built upon the Nix language.

There are other related concepts that might also be relevant:

Nixpkgs: a repository of Nix packages;
NixOS: the Linux distribution built on top of the Nix package manager and Nixpkgs repository;
NixOS (command line utilities): a set of CLI tools used in managing a NixOS installation.