diff --git a/actionpack/lib/action_dispatch/routing/route_set.rb b/actionpack/lib/action_dispatch/routing/route_set.rb
index 19a277a18c..835ba03784 100644
--- a/actionpack/lib/action_dispatch/routing/route_set.rb
+++ b/actionpack/lib/action_dispatch/routing/route_set.rb
@@ -262,7 +262,7 @@ module ActionDispatch
       module MountedHelpers
       end
 
-      def mounted_helpers(name = :app)
+      def mounted_helpers(name = :main_app)
         define_mounted_helper(name) if name
         MountedHelpers
       end
diff --git a/actionpack/test/dispatch/prefix_generation_test.rb b/actionpack/test/dispatch/prefix_generation_test.rb
index 1e2df682b3..26d76557dd 100644
--- a/actionpack/test/dispatch/prefix_generation_test.rb
+++ b/actionpack/test/dispatch/prefix_generation_test.rb
@@ -83,9 +83,9 @@ module TestGenerationPrefix
       end
 
       def url_to_application
-        path = app.url_for( :controller => "outside_engine_generating",
-                            :action => "index",
-                            :only_path => true)
+        path = main_app.url_for(:controller => "outside_engine_generating",
+                                :action => "index",
+                                :only_path => true)
         render :text => path
       end
 
diff --git a/railties/test/application/initializers/frameworks_test.rb b/railties/test/application/initializers/frameworks_test.rb
index 6e9ceb6ef7..da8ae17518 100644
--- a/railties/test/application/initializers/frameworks_test.rb
+++ b/railties/test/application/initializers/frameworks_test.rb
@@ -61,7 +61,7 @@ module ApplicationTests
 
       require "#{app_path}/config/environment"
       assert Foo.method_defined?(:foo_path)
-      assert Foo.method_defined?(:app)
+      assert Foo.method_defined?(:main_app)
       assert_equal ["notify"], Foo.action_methods
     end
 
diff --git a/railties/test/railties/engine_test.rb b/railties/test/railties/engine_test.rb
index fabd561bd2..370f9342e3 100644
--- a/railties/test/railties/engine_test.rb
+++ b/railties/test/railties/engine_test.rb
@@ -437,7 +437,7 @@ module RailtiesTest
           end
 
           def routes_helpers_in_view
-            render :inline => "<%= foo_path %>, <%= app.bar_path %>"
+            render :inline => "<%= foo_path %>, <%= main_app.bar_path %>"
           end
 
           def polymorphic_path_without_namespace
diff --git a/railties/test/railties/mounted_engine_test.rb b/railties/test/railties/mounted_engine_test.rb
index 73b7e5b2e0..b52ced92ec 100644
--- a/railties/test/railties/mounted_engine_test.rb
+++ b/railties/test/railties/mounted_engine_test.rb
@@ -70,14 +70,14 @@ module ApplicationTests
             end
 
             def generate_application_route
-              path = app.url_for(:controller => "/main",
+              path = main_app.url_for(:controller => "/main",
                                  :action => "index",
                                  :only_path => true)
               render :text => path
             end
 
             def application_route_in_view
-              render :inline => "<%= app.root_path %>"
+              render :inline => "<%= main_app.root_path %>"
             end
           end
         end