# Glossary

- [build system]{#gloss-build-system}

  Generic term for software that facilitates the building of software by automating the invocation of compilers, linkers, and other tools.

  Nix can be used as a generic build system.
  It has no knowledge of any particular programming language or toolchain.
  These details are specified in [derivation expressions](#gloss-derivation-expression).

- [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-addressing derivation](#gloss-content-addressing-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).

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

  The industry term for storage and retrieval systems using [content addressing](#gloss-content-address). A Nix store also has [input addressing](#gloss-input-addressed-store-object), and metadata.

- [derivation]{#gloss-derivation}

  A derivation can be thought of as a [pure function](https://en.wikipedia.org/wiki/Pure_function) that produces new [store objects][store object] from existing store objects.

  Derivations are implemented as [operating system processes that run in a sandbox](@docroot@/store/building.md#builder-execution).
  This sandbox by default only allows reading from store objects specified as inputs, and only allows writing to designated [outputs][output] to be [captured as store objects](@docroot@/store/building.md#processing-outputs).

  A derivation is typically specified as a [derivation expression] in the [Nix language], and [instantiated][instantiate] to a [store derivation].
  There are multiple ways of obtaining store objects from store derivatons, collectively called [realisation][realise].


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

  A [derivation] represented as a [store object].

  See [Store Derivation](@docroot@/store/derivation/index.md#store-derivation) for details.


- [directed acyclic graph]{#gloss-directed-acyclic-graph}

  A [directed acyclic graph](https://en.wikipedia.org/wiki/Directed_acyclic_graph) (DAG) is graph whose edges are given a direction ("a to b" is not the same edge as "b to a"), and for which no possible path (created by joining together edges) forms a cycle.

  DAGs are very important to Nix.
  In particular, the non-self-[references][reference] of [store object][store object] form a cycle.

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

  A [store path] which uniquely identifies a [store derivation].

  See [Referencing Store Derivations](@docroot@/store/derivation/index.md#derivation-path) for details.

  Not to be confused with [deriving path].


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

  A description of a [store derivation] using the [`derivation` primitive](./language/derivations.md) in the [Nix language].


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

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

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


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