diff --git a/def/babel.js b/def/babel.js
index c7839080..608ffbc7 100644
--- a/def/babel.js
+++ b/def/babel.js
@@ -50,10 +50,6 @@ def("ParenthesizedExpression")
   .build("expression")
   .field("expression", def("Expression"));
 
-def("ModuleSpecifier")
-  .bases("Specifier")
-  .field("local", def("Identifier"));
-
 def("ImportSpecifier")
   .bases("ModuleSpecifier")
   .build("imported", "local")
diff --git a/def/es6.js b/def/es6.js
index f6ca3145..b73bc07c 100644
--- a/def/es6.js
+++ b/def/es6.js
@@ -178,10 +178,26 @@ def("ClassImplements")
     .field("id", def("Identifier"))
     .field("superClass", or(def("Expression"), null), defaults["null"]);
 
-// Specifier and NamedSpecifier are abstract non-standard types that I
+// Specifier and ModuleSpecifier are abstract non-standard types
 // introduced for definitional convenience.
 def("Specifier").bases("Node");
 
+// This supertype is shared/abused by both def/babel.js and
+// def/esprima.js. In the future, it will be possible to load only one set
+// of definitions appropriate for a given parser, but until then we must
+// rely on default functions to reconcile the conflicting AST formats.
+def("ModuleSpecifier")
+    .bases("Specifier")
+    // This local field is used by Babel/Acorn. It should not technically
+    // be optional in the Babel/Acorn AST format, but it must be optional
+    // in the Esprima AST format.
+    .field("local", or(def("Identifier"), null), defaults["null"])
+    // The id and name fields are used by Esprima. The id field should not
+    // technically be optional in the Esprima AST format, but it must be
+    // optional in the Babel/Acorn AST format.
+    .field("id", or(def("Identifier"), null), defaults["null"])
+    .field("name", or(def("Identifier"), null), defaults["null"]);
+
 def("TaggedTemplateExpression")
     .bases("Expression")
     .build("tag", "quasi")
diff --git a/def/esprima.js b/def/esprima.js
index c807b251..06113909 100644
--- a/def/esprima.js
+++ b/def/esprima.js
@@ -32,16 +32,10 @@ def("ObjectPattern")
         def("SpreadProperty") // Used by Esprima.
     )]);
 
-def("NamedSpecifier")
-    .bases("Specifier")
-    // Note: this abstract type is intentionally not buildable.
-    .field("id", def("Identifier"))
-    .field("name", or(def("Identifier"), null), defaults["null"]);
-
-// Like NamedSpecifier, except type:"ExportSpecifier" and buildable.
+// Like ModuleSpecifier, except type:"ExportSpecifier" and buildable.
 // export {<id [as name]>} [from ...];
 def("ExportSpecifier")
-    .bases("NamedSpecifier")
+    .bases("ModuleSpecifier")
     .build("id", "name");
 
 // export <*> from ...;
@@ -49,10 +43,10 @@ def("ExportBatchSpecifier")
     .bases("Specifier")
     .build();
 
-// Like NamedSpecifier, except type:"ImportSpecifier" and buildable.
+// Like ModuleSpecifier, except type:"ImportSpecifier" and buildable.
 // import {<id [as name]>} from ...;
 def("ImportSpecifier")
-    .bases("NamedSpecifier")
+    .bases("ModuleSpecifier")
     .build("id", "name");
 
 // import <* as id> from ...;