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.


- [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.


- [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.


- [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-addressing-derivation),
  non-[fixed-output](#gloss-fixed-output-derivation)
  derivation.

  See [input-addressing derivation outputs](store/derivation/outputs/input-address.md) for details.

- [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-addressing derivations](#gloss-content-addressing-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.


- [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 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].