Nix Glossary

A way of specifying what you want (the desired state), rather than how to achieve it (imperative steps). Nix is declarative: you describe your system, and Nix figures out how to build it.

The property that building the same Nix expression in different environments (different machines, times) produces identical results. This is Nix's core promise.

A function that always produces the same output for the same input, with no side effects. Nix strives for purity to ensure reproducibility—builds shouldn't depend on random system state.

Once something is built/stored in Nix, it doesn't change. This prevents accidental modifications and enables safe sharing of built artifacts.

A piece of code in the Nix language that evaluates to a value (often a package, configuration, or derivation). It's the unit of specification in Nix.

A Nix object that describes how to build something (source code → compiled binary). It specifies inputs, build steps, and outputs. Often written as stdenv.mkDerivation { ... }.

Nix's equivalent of a dictionary or object: a set of key-value pairs. Written with curly braces.

An ordered collection of values in Nix, written in square brackets.

Embedding expressions inside strings using ${...} syntax. The expression is evaluated and its result is inserted into the string.

A reusable block of code that takes inputs and returns an output. In Nix, all functions are pure.

The official Nix package repository; a massive collection of package definitions and tools. Contains tens of thousands of packages ready to use.

A command that drops you into a shell with specific packages available, without permanently installing them. Useful for isolated dev environments.

A modern Nix file format that defines a project's dependencies and outputs. It's reproducible and self-contained—includes a lock file (flake.lock) pinning all dependencies.

An auto-generated lock file that pins exact versions/revisions of all flake inputs. Ensures reproducibility across machines and time.

A way to customize or extend nixpkgs without modifying it directly. Overlays let you override packages or add new ones.

A Linux distribution configured entirely by Nix. System configuration (packages, services, settings) is declared in a single configuration file.

The main NixOS system configuration file. Defines what packages, services, and settings should be on the system.

A tool for managing user-level configuration (shell settings, dotfiles, user packages) declaratively, on any Linux/macOS system (not just NixOS).

All the dependencies required by a package or system configuration, recursively. Nix tracks closures to ensure everything needed is available.

The directory (usually /nix/store) where Nix keeps all built packages and dependencies. Everything is stored by content hash.

The process of removing unused packages from the Nix store to free disk space. Only removes packages not referenced by any "root".

A unique identifier for a package or derivation based on its inputs and build process. Two identical inputs produce the same hash—used for reproducibility.

A dependency or tool required to build a derivation. Examples: compiler, library, build tool.