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-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.

- [package]{#package}

  A software package; files that belong together for a particular purpose, and metadata.

  Nix represents files as [file system objects][file system object], and how they belong together is encoded as [references][reference] between [store objects][store object] that contain these file system objects.

  The [Nix language] allows denoting packages in terms of [attribute sets](@docroot@/language/types.md#attribute-set) containing:
  - attributes that refer to the files of a package, typically in the form of [derivation outputs](#output),
  - attributes with metadata, such as information about how the package is supposed to be used.

  The exact shape of these attribute sets is up to convention.


- [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.


- [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.


- [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).