# Glossary

- [content address]{#gloss-content-address}

  A
  [*content address*](https://en.wikipedia.org/wiki/Content-addressable_storage)
  is a secure way to reference immutable data.
  The reference is calculated directly from the content of the data being referenced, which means the reference is
  [*tamper proof*](https://en.wikipedia.org/wiki/Tamperproofing)
  --- variations of the data should always calculate to distinct content addresses.

  For how Nix uses content addresses, see:

    - [Content-Addressing File System Objects](@docroot@/store/file-system-object/content-address.md)
    - [Content-Addressing Store Objects](@docroot@/store/store-object/content-address.md)
    - [content-addressed derivation](#gloss-content-addressed-derivation)

  Software Heritage's writing on [*Intrinsic and Extrinsic identifiers*](https://www.softwareheritage.org/2020/07/09/intrinsic-vs-extrinsic-identifiers) is also a good introduction to the value of content-addressing over other referencing schemes.

  Besides content addressing, the Nix store also uses [input addressing](#gloss-input-addressed-store-object).

- [derivation]{#gloss-derivation}

  A description of a build task. The result of a derivation is a
  store object. Derivations declared in Nix expressions are specified
  using the [`derivation` primitive](./language/derivations.md). These are
  translated into low-level *store derivations* (implicitly by
  `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].
  It is the [instantiated][instantiate] form of a derivation.

  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

  Save an evaluated [derivation] as a [store derivation] in the Nix [store].

  See [`nix-instantiate`](./command-ref/nix-instantiate.md), which produces a store derivation from a Nix expression that evaluates to a derivation.

  [instantiate]: #gloss-instantiate

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

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

  This can be achieved by:
  - Fetching a pre-built [store object] from a [substituter]
  - Running the [`builder`](@docroot@/language/derivations.md#attr-builder) executable as specified in the corresponding [derivation]
  - Delegating to a [remote machine](@docroot@/command-ref/conf-file.md#conf-builders) and retrieving the outputs
  <!-- TODO: link [running] to build process page, #8888 -->

  See [`nix-store --realise`](@docroot@/command-ref/nix-store/realise.md) for a detailed description of the algorithm.

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

  [realise]: #gloss-realise

- [content-addressed derivation]{#gloss-content-addressed-derivation}

  A derivation which has the
  [`__contentAddressed`](./language/advanced-attributes.md#adv-attr-__contentAddressed)
  attribute set to `true`.

- [fixed-output derivation]{#gloss-fixed-output-derivation} (FOD)

  A [derivation] where a cryptographic hash of the [output] is determined in advance using the [`outputHash`](./language/advanced-attributes.md#adv-attr-outputHash) attribute, and where the [`builder`](@docroot@/language/derivations.md#attr-builder) executable has access to the network.

- [store]{#gloss-store}

  A collection of [store objects][store object], with operations to manipulate that collection.
  See [Nix Store](./store/index.md) for details.

  There are many types of stores, see [Store Types](./store/types/index.md) for details.

  [store]: #gloss-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`

  See [Store Path](@docroot@/store/store-path.md) for details.

  [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@/store/file-system-object.md) for details.

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

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

  Part of the contents of a [store].

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

  See [Store Object](@docroot@/store/store-object.md) for details.

  [store object]: #gloss-store-object

- [IFD]{#gloss-ifd}

  [Import From Derivation](./language/import-from-derivation.md)

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

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

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

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

  See [Content-Addressing Store Objects](@docroot@/store/store-object/content-address.md) for details.

- [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 Nix 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.

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

  [An experimental feature](#@docroot@/development/experimental-features.md#xp-feature-impure-derivations) that allows derivations to be explicitly marked as impure,
  so that they are always rebuilt, and their outputs not reused by subsequent calls to realise them.

- [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 syntactically valid use of the [Nix language].

  > **Example**
  >
  > The contents of a `.nix` file form a Nix expression.

  Nix expressions specify [derivations][derivation], which are [instantiated][instantiate] into the Nix store as [store derivations][store derivation].
  These derivations can then be [realised][realise] to produce [outputs][output].

  > **Example**
  >
  > Building and deploying software using Nix entails writing Nix expressions as a high-level description of packages and compositions thereof.

- [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].
  See [the `outputs` argument to the `derivation` function](@docroot@/language/derivations.md#attr-outputs) for details.

  [output]: #gloss-output

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

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

  [output path]: #gloss-output-path

- [output closure]{#gloss-output-closure}\
  The [closure] of an [output path]. It only contains what is [reachable] from the output.

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

  Deriving paths are a way to refer to [store objects][store object] that ar not yet [realised][realise].
  This is necessary because, in general and particularly for [content-addressed derivations][content-addressed derivation], the [output path] of an [output] is not known in advance.
  There are two forms:

  - *constant*: just a [store path]
    It can be made [valid][validity] by copying it into the store: from the evaluator, command line interface or another store.

  - *output*: a pair of a [store path] to a [derivation] and an [output] name.

- [deriver]{#gloss-deriver}

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

  The deriver for an output path can be queried with the `--deriver` option to
  [`nix-store --query`](@docroot@/command-ref/nix-store/query.md).

- [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
  [local store]: @docroot@/store/types/local-store.md

- [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.

- [Nix Archive (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`.

  See [Nix Archive](store/file-system-object/content-address.html#serial-nix-archive) for details.

- [`∅`]{#gloss-emtpy-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.

- [package]{#package}

  1. A software package; a collection of files and other data.

  2. A [package attribute set].

- [package attribute set]{#package-attribute-set}

  An [attribute set](@docroot@/language/types.md#attribute-set) containing the attribute `type = "derivation";` (derivation for historical reasons), as well as other attributes, such as
  - attributes that refer to the files of a [package], typically in the form of [derivation outputs](#output),
  - attributes that declare something about how the package is supposed to be installed or used,
  - other metadata or arbitrary attributes.

  [package attribute set]: #package-attribute-set

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

- [base directory]{#gloss-base-directory}

  The location from which relative paths are resolved.

  - For expressions in a file, the base directory is the directory containing that file.
    This is analogous to the directory of a [base URL](https://datatracker.ietf.org/doc/html/rfc1808#section-3.3).
    <!-- which is sufficient for resolving non-empty URLs -->

  <!--
    The wording here may look awkward, but it's for these reasons:
      * "with --expr": it's a flag, and not an option with an accompanying value
      * "written in": the expression itself must be written as an argument,
        whereas the more natural "passed as an argument" allows an interpretation
        where the expression could be passed by file name.
    -->
  - For expressions written in command line arguments with [`--expr`](@docroot@/command-ref/opt-common.html#opt-expr), the base directory is the current working directory.

  [base directory]: #gloss-base-directory

- [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@/development/experimental-features.md).


[Nix language]: ./language/index.md
