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. 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 normal the 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 components and compositions thereof. Deploying software using Nix entails writing Nix expressions for your components. Nix expressions are translated to derivations that are stored in the Nix store. These derivations can then be built. closure The closure of a store path is the set of store paths that are directly or indirectly “reachable” from that store path. For instance, if the store object at path P contains a reference to path Q, then Q is in the closure of P. 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. 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.