Nix is a purely functional package manager. Think of it like a cross-platform alternative to homebrew, apt-get, stack—one that can both install packages and manage their development and deployment environments, in a purely functional manner (thus entirely reproducible).

nilkn explains the terminology,

“nix” as a term refers to a system of multiple components that work together to achieve that goal. There’s the package manager itself (called Nix), the language that build instructions are written in (also called Nix), an existing ecosystem of predefined packages (called nixpkgs), and an optional Linux distro that uses Nix the package manager as its package manager (called NixOS).

Nix has a learning curve (which discourages some users), although Nix delights users.