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 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 [derivation expressions][derivation expression], 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 to describe [packages][package] and compositions thereof.

- [reference]{#gloss-reference}

  An edge from one [store object] to another.

  See [References](@docroot@/store/store-object.md#references) for details.


  See [References](@docroot@/store/store-object.md#references) for details.

- [reachable]{#gloss-reachable}

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

  See [References](@docroot@/store/store-object.md#references) for details.

- [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`.

  See [References](@docroot@/store/store-object.md#references) for details.


- [requisite]{#gloss-requisite}

  A store object [reachable] by a path (chain of references) from a given [store object].
  The [closure] is the set of requisites.

  See [References](@docroot@/store/store-object.md#references) for details.

- [referrer]{#gloss-reference}

  A reversed edge from one [store object] to another.

- [output]{#gloss-output}

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


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

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


- [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 might not yet be [realised][realise].

  See [Deriving Path](./store/derivation/index.md#deriving-path) for details.

  Not to be confused with [derivation path].

- [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].