diff --git a/actionpack/test/controller/parameters/serialization_test.rb b/actionpack/test/controller/parameters/serialization_test.rb
index 2e315c9f6f..abc83bfe48 100644
--- a/actionpack/test/controller/parameters/serialization_test.rb
+++ b/actionpack/test/controller/parameters/serialization_test.rb
@@ -13,7 +13,7 @@ class ParametersSerializationTest < ActiveSupport::TestCase
     ActionController::Parameters.permit_all_parameters = @old_permitted_parameters
   end
 
-  test "yaml serialization" do
+  test "YAML serialization" do
     params = ActionController::Parameters.new(key: :value)
     yaml_dump = YAML.dump(params)
     assert_match("--- !ruby/object:ActionController::Parameters", yaml_dump)
@@ -21,7 +21,7 @@ class ParametersSerializationTest < ActiveSupport::TestCase
     assert_match("permitted: false", yaml_dump)
   end
 
-  test "yaml deserialization" do
+  test "YAML deserialization" do
     params = ActionController::Parameters.new(key: :value)
     payload = YAML.dump(params)
     roundtripped = YAML.respond_to?(:unsafe_load) ? YAML.unsafe_load(payload) : YAML.load(payload)
@@ -30,7 +30,7 @@ class ParametersSerializationTest < ActiveSupport::TestCase
     assert_not_predicate roundtripped, :permitted?
   end
 
-  test "yaml backwardscompatible with psych 2.0.8 format" do
+  test "YAML backwardscompatible with psych 2.0.8 format" do
     payload = <<~end_of_yaml
       --- !ruby/hash:ActionController::Parameters
       key: :value
@@ -41,7 +41,7 @@ class ParametersSerializationTest < ActiveSupport::TestCase
     assert_not_predicate params, :permitted?
   end
 
-  test "yaml backwardscompatible with psych 2.0.9+ format" do
+  test "YAML backwardscompatible with psych 2.0.9+ format" do
     payload = <<~end_of_yaml
       --- !ruby/hash-with-ivars:ActionController::Parameters
       elements:
diff --git a/actionpack/test/controller/parameters_integration_test.rb b/actionpack/test/controller/parameters_integration_test.rb
index 2cf173d961..20454e9330 100644
--- a/actionpack/test/controller/parameters_integration_test.rb
+++ b/actionpack/test/controller/parameters_integration_test.rb
@@ -11,7 +11,7 @@ end
 class ActionControllerParametersIntegrationTest < ActionController::TestCase
   tests IntegrationController
 
-  test "parameters can be serialized as yaml" do
+  test "parameters can be serialized as YAML" do
     post :yaml_params, params: { person: { name: "Mjallo!" } }
     expected = <<~YAML
 --- !ruby/object:ActionController::Parameters
diff --git a/activerecord/lib/active_record/connection_adapters/abstract/connection_pool/reaper.rb b/activerecord/lib/active_record/connection_adapters/abstract/connection_pool/reaper.rb
index f5351af497..0bff96e171 100644
--- a/activerecord/lib/active_record/connection_adapters/abstract/connection_pool/reaper.rb
+++ b/activerecord/lib/active_record/connection_adapters/abstract/connection_pool/reaper.rb
@@ -11,7 +11,7 @@ module ActiveRecord
       # the connection pool.
       #
       # Configure the frequency by setting +reaping_frequency+ in your database
-      # yaml file (default 60 seconds).
+      # YAML file (default 60 seconds).
       class Reaper
         attr_reader :pool, :frequency
 
diff --git a/activesupport/test/configuration_file_test.rb b/activesupport/test/configuration_file_test.rb
index 49f83c332b..6a14f273fe 100644
--- a/activesupport/test/configuration_file_test.rb
+++ b/activesupport/test/configuration_file_test.rb
@@ -3,7 +3,7 @@
 require_relative "abstract_unit"
 
 class ConfigurationFileTest < ActiveSupport::TestCase
-  test "backtrace contains yaml path" do
+  test "backtrace contains YAML path" do
     Tempfile.create do |file|
       file.write("wrong: <%= foo %>")
       file.rewind
@@ -16,7 +16,7 @@ class ConfigurationFileTest < ActiveSupport::TestCase
     end
   end
 
-  test "backtrace contains yaml path (when Pathname given)" do
+  test "backtrace contains YAML path (when Pathname given)" do
     Tempfile.create do |file|
       file.write("wrong: <%= foo %>")
       file.rewind
diff --git a/guides/source/action_cable_overview.md b/guides/source/action_cable_overview.md
index d4228c4321..36526370b2 100644
--- a/guides/source/action_cable_overview.md
+++ b/guides/source/action_cable_overview.md
@@ -723,7 +723,7 @@ The Redis adapter requires users to provide a URL pointing to the Redis server.
 Additionally, a `channel_prefix` may be provided to avoid channel name collisions
 when using the same Redis server for multiple applications. See the [Redis PubSub documentation](https://redis.io/topics/pubsub#database-amp-scoping) for more details.
 
-The Redis adapter also supports SSL/TLS connections. The required SSL/TLS parameters can be passed in `ssl_params` key in the configuration yaml file.
+The Redis adapter also supports SSL/TLS connections. The required SSL/TLS parameters can be passed in `ssl_params` key in the configuration YAML file.
 
 ```
 production:
diff --git a/railties/test/application/configuration_test.rb b/railties/test/application/configuration_test.rb
index 0e86f547b3..729089b7cd 100644
--- a/railties/test/application/configuration_test.rb
+++ b/railties/test/application/configuration_test.rb
@@ -1960,7 +1960,7 @@ module ApplicationTests
       assert_equal true, Rails.application.config.action_mailer.show_previews
     end
 
-    test "config_for loads custom configuration from yaml accessible as symbol or string" do
+    test "config_for loads custom configuration from YAML accessible as symbol or string" do
       set_custom_config <<~RUBY
         development:
           foo: "bar"
@@ -1972,7 +1972,7 @@ module ApplicationTests
       assert_equal "bar", Rails.application.config.my_custom_config["foo"]
     end
 
-    test "config_for loads nested custom configuration from yaml as symbol keys" do
+    test "config_for loads nested custom configuration from YAML as symbol keys" do
       set_custom_config <<~RUBY
         development:
           foo: