diff --git a/Source/cmDefinitions.cxx b/Source/cmDefinitions.cxx
index abb46b3332..58500c94c8 100644
--- a/Source/cmDefinitions.cxx
+++ b/Source/cmDefinitions.cxx
@@ -61,6 +61,11 @@ void cmDefinitions::Set(const std::string& key, const char* value)
     }
 }
 
+void cmDefinitions::Erase(const std::string& key)
+{
+  this->Map.erase(key);
+}
+
 //----------------------------------------------------------------------------
 std::set<std::string> cmDefinitions::LocalKeys() const
 {
diff --git a/Source/cmDefinitions.h b/Source/cmDefinitions.h
index 83cd0d9ce7..13f885df8b 100644
--- a/Source/cmDefinitions.h
+++ b/Source/cmDefinitions.h
@@ -40,6 +40,8 @@ public:
   /** Set (or unset if null) a value associated with a key.  */
   void Set(const std::string& key, const char* value);
 
+  void Erase(const std::string& key);
+
   /** Get the set of all local keys.  */
   std::set<std::string> LocalKeys() const;