# Glossary

- derivation
  A description of a build action. The result of a derivation is a store object. Derivations are typically specified in Nix expressions using the `derivation` primitive. These are translated into low-level store derivations (implicitly by `nix-env` and `nix-build`, or explicitly by `nix-instantiate`).

- store
  The location in the file system where store objects live. Typically `/nix/store`.

- store path
  The location in the file system of a store object, i.e., an immediate child of the Nix store directory.

- store object
  A file that is an immediate child of the Nix store directory. These can be regular files, but also entire directory trees. Store objects can be sources (objects copied from outside of the store), derivation outputs (objects produced by running a build action), or derivations (files describing a build action).

- substitute
  A substitute is a command invocation stored in the Nix database that describes how to build a store object, bypassing the normal build mechanism (i.e., derivations). Typically, the substitute builds the store object by downloading a pre-built version of the store object from some server.

- purity
  The assumption that equal Nix derivations when run always produce the same output. This cannot be guaranteed in general (e.g., a builder can rely on external inputs such as the network or the system time) but the Nix model assumes it.

- Nix expression
  A high-level description of software packages and compositions thereof. Deploying software using Nix entails writing Nix expressions for your packages. Nix expressions are translated to derivations that are stored in the Nix store. These derivations can then be built.

- reference
  A store path `P` is said to have a reference to a store path `Q` if the store object at `P` contains the path `Q` somewhere. The references of a store path are the set of store paths to which it has a reference.

  A derivation can reference other derivations and sources (but not output paths), whereas an output path only references other output paths.

- reachable
  A store path `Q` is reachable from another store path `P` if `Q` is in the closure of the references relation.

- closure
  The closure of a store path is the set of store paths that are directly or indirectly “reachable” from that store path; that is, it’s the closure of the path under the references relation. For a package, the closure of its derivation is equivalent to the build-time dependencies, while the closure of its output path is equivalent to its runtime dependencies. For correct deployment it is necessary to deploy whole closures, since otherwise at runtime files could be missing. The command `nix-store -qR` prints out closures of store paths.

  As an example, if the store object at path `P` contains a reference to path `Q`, then `Q` is in the closure of `P`. Further, if `Q` references `R` then `R` is also in the closure of `P`.

- output path
  A store path produced by a derivation.

- deriver
  The deriver of an output path is the store derivation that built it.

- validity
  A store path is considered valid if it exists in the file system, is listed in the Nix database as being valid, and if all paths in its closure are also valid.

- user environment
  An automatically generated store object that consists of a set of symlinks to “active” applications, i.e., other store paths. These are generated automatically by `nix-env`. See profiles.

- profile
  A symlink to the current user environment of a user, e.g., `/nix/var/nix/profiles/default`.

- NAR
  A Nix ARchive. This is a serialisation of a path in the Nix store. It can contain regular files, directories and symbolic links. NARs are generated and unpacked using `nix-store --dump` and `nix-store --restore`.

- `∅`
  The empty set symbol. In the context of profile history, this denotes a package is not present in a particular version of the profile.

- `ε`
  The epsilon symbol. In the context of a package, this means the version is empty. More precisely, the derivation does not have a version attribute.