# Glossary

- [derivation]{#gloss-derivation}

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

  [derivation]: #gloss-derivation

- [store derivation]{#gloss-store-derivation}

  A [derivation] represented as a `.drv` file in the [store].
  It has a [store path], like any [store object].

  Example: `/nix/store/g946hcz4c8mdvq2g8vxx42z51qb71rvp-git-2.38.1.drv`

  See [`nix derivation show`](./command-ref/new-cli/nix3-derivation-show.md) (experimental) for displaying the contents of store derivations.

  [store derivation]: #gloss-store-derivation

- [instantiate]{#gloss-instantiate}, instantiation

  Translate a [derivation] into a [store derivation].

  See [`nix-instantiate`](./command-ref/nix-instantiate.md).

  [instantiate]: #gloss-instantiate

- [realise]{#gloss-realise}, realisation

  Ensure a [store path] is [valid][validity].

  This means either running the [`builder`](@docroot@/language/derivations.md#attr-builder) executable as specified in the corresponding [derivation], or fetching a pre-built [store object] from a [substituter], or delegating to a [remote builder](@docroot@/advanced-topics/distributed-builds.html) and retrieving the outputs. <!-- TODO: link [running] to build process page, #8888 -->

  See [`nix-build`](./command-ref/nix-build.md) and [`nix-store --realise`](@docroot@/command-ref/nix-store/realise.md).

  See [`nix build`](./command-ref/new-cli/nix3-build.md) (experimental).

  [realise]: #gloss-realise

- [fixed-output derivation]{#gloss-fixed-output-derivation}

  A derivation which includes the
  [`outputHash`](./language/advanced-attributes.md#adv-attr-outputHash) attribute.

- [store]{#gloss-store}

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

  From   the  perspective   of   the  location   where  Lix   is
  invoked, the  Nix store can be  referred to
  as a "_local_" or a "_remote_" one:

  + A [local store]{#gloss-local-store} exists on the filesystem of
    the machine where Lix is  invoked. You can use other
    local stores  by passing  the `--store` flag  to the
    `nix` command.  Local stores can be used for building derivations.

  + A  *remote store*  exists  anywhere  other than  the
    local  filesystem. One  example is  the `/nix/store`
    directory on another machine,  accessed via `ssh` or
    served by the `nix-serve` Perl script.

  [store]: #gloss-store
  [local store]: #gloss-local-store

- [chroot store]{#gloss-chroot-store}

  A [local store] whose canonical path is anything other than `/nix/store`.

- [binary cache]{#gloss-binary-cache}

  A *binary cache* is a Nix store which uses a different format: its
  metadata and signatures are kept in `.narinfo` files rather than in a
  [Nix database]. This different format simplifies serving store objects
  over the network, but cannot host builds. Examples of binary caches
  include S3 buckets and the [NixOS binary cache](https://cache.nixos.org).

- [store path]{#gloss-store-path}

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

  Example: `/nix/store/a040m110amc4h71lds2jmr8qrkj2jhxd-git-2.38.1`

  [store path]: #gloss-store-path

- [file system object]{#gloss-file-system-object}

  The Nix data model for representing simplified file system data.

  See [File System Object](@docroot@/architecture/file-system-object.md) for details.

  [file system object]: #gloss-store-path

- [store object]{#gloss-store-object}


  A store object consists of a [file system object], [reference]s to other store objects, and other metadata.
  It can be referred to by a [store path].

  [store object]: #gloss-store-object

- [input-addressed store object]{#gloss-input-addressed-store-object}

  A store object produced by building a
  non-[fixed-output](#gloss-fixed-output-derivation)
  derivation.

- [output-addressed store object]{#gloss-output-addressed-store-object}

  A [store object] whose [store path] is determined by its contents.
  This includes derivations and the outputs of [fixed-output derivations](#gloss-fixed-output-derivation).

- [substitute]{#gloss-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.

- [substituter]{#gloss-substituter}

  An additional [store]{#gloss-store} from which Lix can obtain store objects instead of building them.
  Often the substituter is a [binary cache](#gloss-binary-cache), but any store can serve as substituter.

  See the [`substituters` configuration option](./command-ref/conf-file.md#conf-substituters) for details.

  [substituter]: #gloss-substituter

- [purity]{#gloss-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 database]{#gloss-nix-database}

  An SQlite database to track [reference]s between [store object]s.
  This is an implementation detail of the [local store].

  Default location: `/nix/var/nix/db`.

  [Nix database]: #gloss-nix-database

- [Nix expression]{#gloss-nix-expression}

  A high-level description of software packages and compositions
  thereof. Deploying software using Lix 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]{#gloss-reference}

  A [store object] `O` is said to have a *reference* to a store object `P` if a [store path] to `P` appears in the contents of `O`.

  Store objects can refer to both other store objects and themselves.
  References from a store object to itself are called *self-references*.
  References other than a self-reference must not form a cycle.

  [reference]: #gloss-reference

- [reachable]{#gloss-reachable}

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

- [closure]{#gloss-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 --query --requisites ` prints out
  closures of store paths.

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

  [closure]: #gloss-closure

- [output]{#gloss-output}

  A [store object] produced by a [derivation].

  [output]: #gloss-output

- [output path]{#gloss-output-path}

  The [store path] to the [output] of a [derivation].

  [output path]: #gloss-output-path

- [deriver]{#gloss-deriver}

  The [store derivation] that produced an [output path].

- [validity]{#gloss-validity}

  A store path is valid if all [store object]s in its [closure] can be read from the [store].

  For a [local store], this means:
  - The store path leads to an existing [store object] in that [store].
  - The store path is listed in the [Nix database] as being valid.
  - All paths in the store path's [closure] are valid.

  [validity]: #gloss-validity

- [user environment]{#gloss-user-env}

  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`](./command-ref/nix-env.md). See *profiles*.

- [profile]{#gloss-profile}

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

- [installable]{#gloss-installable}

  Something that can be realised in the Nix store.

  See [installables](./command-ref/new-cli/nix.md#installables) for [`nix` commands](./command-ref/new-cli/nix.md) (experimental) for details.

- [NAR]{#gloss-nar}

  A *N*ix *AR*chive. 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`.

- [`∅`]{#gloss-empty-set}

  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`]{#gloss-epsilon}

  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.

- [string interpolation]{#gloss-string-interpolation}

  Expanding expressions enclosed in `${ }` within a [string], [path], or [attribute name].

  See [String interpolation](./language/string-interpolation.md) for details.

  [string]: ./language/values.md#type-string
  [path]: ./language/values.md#type-path
  [attribute name]: ./language/values.md#attribute-set

- [experimental feature]{#gloss-experimental-feature}

  Not yet stabilized functionality guarded by named experimental feature flags.
  These flags are enabled or disabled with the [`experimental-features`](./command-ref/conf-file.html#conf-experimental-features) setting.

  See the contribution guide on the [purpose and lifecycle of experimental feaures](@docroot@/contributing/experimental-features.md).
