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+# Glossary
+
+  - derivation  
+    A description of a build action. The result of a derivation is a
+    store object. Derivations are typically specified in Nix expressions
+    using the [`derivation` primitive](expressions/derivations.md). These are
+    translated into low-level *store derivations* (implicitly by
+    `nix-env` and `nix-build`, or explicitly by `nix-instantiate`).
+
+  - store  
+    The location in the file system where store objects live. Typically
+    `/nix/store`.
+
+  - store path  
+    The location in the file system of a store object, i.e., an
+    immediate child of the Nix store directory.
+
+  - store object  
+    A file that is an immediate child of the Nix store directory. These
+    can be regular files, but also entire directory trees. Store objects
+    can be sources (objects copied from outside of the store),
+    derivation outputs (objects produced by running a build action), or
+    derivations (files describing a build action).
+
+  - 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.
+
+  - 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.
+
+  - Nix expression  
+    A high-level description of software packages and compositions
+    thereof. Deploying software using Nix entails writing Nix
+    expressions for your packages. Nix expressions are translated to
+    derivations that are stored in the Nix store. These derivations can
+    then be built.
+
+  - reference  
+    A store path `P` is said to have a reference to a store path `Q` if
+    the store object at `P` contains the path `Q` somewhere. The
+    *references* of a store path are the set of store paths to which it
+    has a reference.
+    
+    A derivation can reference other derivations and sources (but not
+    output paths), whereas an output path only references other output
+    paths.
+
+  - reachable  
+    A store path `Q` is reachable from another store path `P` if `Q`
+    is in the *closure* of the *references* relation.
+
+  - 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 -qR` prints out
+    closures of store paths.
+    
+    As an example, if the store object at path `P` contains a reference
+    to path `Q`, then `Q` is in the closure of `P`. Further, if `Q`
+    references `R` then `R` is also in the closure of `P`.
+
+  - output path  
+    A store path produced by a derivation.
+
+  - deriver  
+    The deriver of an *output path* is the store
+    derivation that built it.
+
+  - validity  
+    A store path is considered *valid* if it exists in the file system,
+    is listed in the Nix database as being valid, and if all paths in
+    its closure are also valid.
+
+  - user environment  
+    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  
+    A symlink to the current *user environment* of a user, e.g.,
+    `/nix/var/nix/profiles/default`.
+
+  - 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`.