diff --git a/doc/manual/src/glossary.md b/doc/manual/src/glossary.md
index 66e4628c0..88916bb50 100644
--- a/doc/manual/src/glossary.md
+++ b/doc/manual/src/glossary.md
@@ -295,6 +295,25 @@
   [path]: ./language/values.md#type-path
   [attribute name]: ./language/values.md#attribute-set
 
+- [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.
+
+  [base directory]: #gloss-base-directory
+
 - [experimental feature]{#gloss-experimental-feature}
 
   Not yet stabilized functionality guarded by named experimental feature flags.
diff --git a/src/libcmd/common-eval-args.hh b/src/libcmd/common-eval-args.hh
index 25ce5b9da..0cb9ddbca 100644
--- a/src/libcmd/common-eval-args.hh
+++ b/src/libcmd/common-eval-args.hh
@@ -38,6 +38,9 @@ private:
     std::map<std::string, AutoArg> autoArgs;
 };
 
+/**
+ * @param baseDir Optional [base directory](https://nixos.org/manual/nix/unstable/glossary#gloss-base-directory)
+ */
 SourcePath lookupFileArg(EvalState & state, std::string_view s, const Path * baseDir = nullptr);
 
 }
diff --git a/src/libexpr/flake/flakeref.hh b/src/libexpr/flake/flakeref.hh
index 5d78f49b6..04c812ed0 100644
--- a/src/libexpr/flake/flakeref.hh
+++ b/src/libexpr/flake/flakeref.hh
@@ -68,24 +68,39 @@ struct FlakeRef
 
 std::ostream & operator << (std::ostream & str, const FlakeRef & flakeRef);
 
+/**
+ * @param baseDir Optional [base directory](https://nixos.org/manual/nix/unstable/glossary#gloss-base-directory)
+ */
 FlakeRef parseFlakeRef(
     const std::string & url,
     const std::optional<Path> & baseDir = {},
     bool allowMissing = false,
     bool isFlake = true);
 
+/**
+ * @param baseDir Optional [base directory](https://nixos.org/manual/nix/unstable/glossary#gloss-base-directory)
+ */
 std::optional<FlakeRef> maybeParseFlake(
     const std::string & url, const std::optional<Path> & baseDir = {});
 
+/**
+ * @param baseDir Optional [base directory](https://nixos.org/manual/nix/unstable/glossary#gloss-base-directory)
+ */
 std::pair<FlakeRef, std::string> parseFlakeRefWithFragment(
     const std::string & url,
     const std::optional<Path> & baseDir = {},
     bool allowMissing = false,
     bool isFlake = true);
 
+/**
+ * @param baseDir Optional [base directory](https://nixos.org/manual/nix/unstable/glossary#gloss-base-directory)
+ */
 std::optional<std::pair<FlakeRef, std::string>> maybeParseFlakeRefWithFragment(
     const std::string & url, const std::optional<Path> & baseDir = {});
 
+/**
+ * @param baseDir Optional [base directory](https://nixos.org/manual/nix/unstable/glossary#gloss-base-directory)
+ */
 std::tuple<FlakeRef, std::string, ExtendedOutputsSpec> parseFlakeRefWithFragmentAndExtendedOutputsSpec(
     const std::string & url,
     const std::optional<Path> & baseDir = {},
diff --git a/src/libutil/args.hh b/src/libutil/args.hh
index 4b2e1d960..7759b74a9 100644
--- a/src/libutil/args.hh
+++ b/src/libutil/args.hh
@@ -41,7 +41,7 @@ public:
     virtual std::string doc() { return ""; }
 
     /**
-     * @brief Get the base directory for the command.
+     * @brief Get the [base directory](https://nixos.org/manual/nix/unstable/glossary#gloss-base-directory) for the command.
      *
      * @return Generally the working directory, but in case of a shebang
      *         interpreter, returns the directory of the script.