Reproducible
Nix builds packages in isolation from each other. This ensures that they
are reproducible and don't have undeclared dependencies, so if a package
works on one machine, it will also work on another.

Declarative
Nix makes it trivial to share development and build environments for your
projects, regardless of what programming languages and tools you’re
using.

Reliable
Nix ensures that installing or upgrading one package cannot break other
packages. It allows you to roll back to previous versions, and ensures that
no package is in an inconsistent state during an upgrade.

NixOS Manual - NixOS is a Linux distribution based on Nix package manager

Nixpkgs Manual - The Nix Packages collection (Nixpkgs) is a set of thousands of packages for the Nix package manager and NixOS Linux distribution