diff --git a/src/compiler/commandLineParser.ts b/src/compiler/commandLineParser.ts
index 08215b5bcfe..09107e83df8 100644
--- a/src/compiler/commandLineParser.ts
+++ b/src/compiler/commandLineParser.ts
@@ -727,7 +727,7 @@ namespace ts {
             type: "boolean",
             affectsSemanticDiagnostics: true,
             category: Diagnostics.Type_Checking,
-            description: Diagnostics.Add_undefined_to_a_type_when_accessed_using_an_index
+            description: Diagnostics.Ensure_overriding_members_in_derived_classes_are_marked_with_an_override_modifier
         },
         {
             name: "noPropertyAccessFromIndexSignature",
diff --git a/tests/baselines/reference/tsConfig/Default initialized TSConfig/tsconfig.json b/tests/baselines/reference/tsConfig/Default initialized TSConfig/tsconfig.json
index fabb4d8bc80..a936136ad6a 100644
--- a/tests/baselines/reference/tsConfig/Default initialized TSConfig/tsconfig.json	
+++ b/tests/baselines/reference/tsConfig/Default initialized TSConfig/tsconfig.json	
@@ -88,7 +88,7 @@
     // "noImplicitReturns": true,                        /* Enable error reporting for codepaths that do not explicitly return in a function. */
     // "noFallthroughCasesInSwitch": true,               /* Enable error reporting for fallthrough cases in switch statements. */
     // "noUncheckedIndexedAccess": true,                 /* Include 'undefined' in index signature results */
-    // "noImplicitOverride": true,                       /* Add `undefined` to a type when accessed using an index. */
+    // "noImplicitOverride": true,                       /* Ensure overriding members in derived classes are marked with an override modifier. */
     // "noPropertyAccessFromIndexSignature": true,       /* Enforces using indexed accessors for keys declared using an indexed type */
     // "allowUnusedLabels": true,                        /* Disable error reporting for unused labels. */
     // "allowUnreachableCode": true,                     /* Disable error reporting for unreachable code. */
diff --git a/tests/baselines/reference/tsConfig/Initialized TSConfig with advanced options/tsconfig.json b/tests/baselines/reference/tsConfig/Initialized TSConfig with advanced options/tsconfig.json
index d7ab4c78b3b..909d91ad019 100644
--- a/tests/baselines/reference/tsConfig/Initialized TSConfig with advanced options/tsconfig.json	
+++ b/tests/baselines/reference/tsConfig/Initialized TSConfig with advanced options/tsconfig.json	
@@ -88,7 +88,7 @@
     // "noImplicitReturns": true,                        /* Enable error reporting for codepaths that do not explicitly return in a function. */
     // "noFallthroughCasesInSwitch": true,               /* Enable error reporting for fallthrough cases in switch statements. */
     // "noUncheckedIndexedAccess": true,                 /* Include 'undefined' in index signature results */
-    // "noImplicitOverride": true,                       /* Add `undefined` to a type when accessed using an index. */
+    // "noImplicitOverride": true,                       /* Ensure overriding members in derived classes are marked with an override modifier. */
     // "noPropertyAccessFromIndexSignature": true,       /* Enforces using indexed accessors for keys declared using an indexed type */
     // "allowUnusedLabels": true,                        /* Disable error reporting for unused labels. */
     // "allowUnreachableCode": true,                     /* Disable error reporting for unreachable code. */
diff --git a/tests/baselines/reference/tsConfig/Initialized TSConfig with boolean value compiler options/tsconfig.json b/tests/baselines/reference/tsConfig/Initialized TSConfig with boolean value compiler options/tsconfig.json
index 3641adac589..940e1d42111 100644
--- a/tests/baselines/reference/tsConfig/Initialized TSConfig with boolean value compiler options/tsconfig.json	
+++ b/tests/baselines/reference/tsConfig/Initialized TSConfig with boolean value compiler options/tsconfig.json	
@@ -88,7 +88,7 @@
     // "noImplicitReturns": true,                        /* Enable error reporting for codepaths that do not explicitly return in a function. */
     // "noFallthroughCasesInSwitch": true,               /* Enable error reporting for fallthrough cases in switch statements. */
     // "noUncheckedIndexedAccess": true,                 /* Include 'undefined' in index signature results */
-    // "noImplicitOverride": true,                       /* Add `undefined` to a type when accessed using an index. */
+    // "noImplicitOverride": true,                       /* Ensure overriding members in derived classes are marked with an override modifier. */
     // "noPropertyAccessFromIndexSignature": true,       /* Enforces using indexed accessors for keys declared using an indexed type */
     // "allowUnusedLabels": true,                        /* Disable error reporting for unused labels. */
     // "allowUnreachableCode": true,                     /* Disable error reporting for unreachable code. */
diff --git a/tests/baselines/reference/tsConfig/Initialized TSConfig with enum value compiler options/tsconfig.json b/tests/baselines/reference/tsConfig/Initialized TSConfig with enum value compiler options/tsconfig.json
index c17c9e888bc..9e5b2f31468 100644
--- a/tests/baselines/reference/tsConfig/Initialized TSConfig with enum value compiler options/tsconfig.json	
+++ b/tests/baselines/reference/tsConfig/Initialized TSConfig with enum value compiler options/tsconfig.json	
@@ -88,7 +88,7 @@
     // "noImplicitReturns": true,                        /* Enable error reporting for codepaths that do not explicitly return in a function. */
     // "noFallthroughCasesInSwitch": true,               /* Enable error reporting for fallthrough cases in switch statements. */
     // "noUncheckedIndexedAccess": true,                 /* Include 'undefined' in index signature results */
-    // "noImplicitOverride": true,                       /* Add `undefined` to a type when accessed using an index. */
+    // "noImplicitOverride": true,                       /* Ensure overriding members in derived classes are marked with an override modifier. */
     // "noPropertyAccessFromIndexSignature": true,       /* Enforces using indexed accessors for keys declared using an indexed type */
     // "allowUnusedLabels": true,                        /* Disable error reporting for unused labels. */
     // "allowUnreachableCode": true,                     /* Disable error reporting for unreachable code. */
diff --git a/tests/baselines/reference/tsConfig/Initialized TSConfig with files options/tsconfig.json b/tests/baselines/reference/tsConfig/Initialized TSConfig with files options/tsconfig.json
index 1d8029a48a5..5553575444d 100644
--- a/tests/baselines/reference/tsConfig/Initialized TSConfig with files options/tsconfig.json	
+++ b/tests/baselines/reference/tsConfig/Initialized TSConfig with files options/tsconfig.json	
@@ -88,7 +88,7 @@
     // "noImplicitReturns": true,                        /* Enable error reporting for codepaths that do not explicitly return in a function. */
     // "noFallthroughCasesInSwitch": true,               /* Enable error reporting for fallthrough cases in switch statements. */
     // "noUncheckedIndexedAccess": true,                 /* Include 'undefined' in index signature results */
-    // "noImplicitOverride": true,                       /* Add `undefined` to a type when accessed using an index. */
+    // "noImplicitOverride": true,                       /* Ensure overriding members in derived classes are marked with an override modifier. */
     // "noPropertyAccessFromIndexSignature": true,       /* Enforces using indexed accessors for keys declared using an indexed type */
     // "allowUnusedLabels": true,                        /* Disable error reporting for unused labels. */
     // "allowUnreachableCode": true,                     /* Disable error reporting for unreachable code. */
diff --git a/tests/baselines/reference/tsConfig/Initialized TSConfig with incorrect compiler option value/tsconfig.json b/tests/baselines/reference/tsConfig/Initialized TSConfig with incorrect compiler option value/tsconfig.json
index d5597eb32de..6b2e1264011 100644
--- a/tests/baselines/reference/tsConfig/Initialized TSConfig with incorrect compiler option value/tsconfig.json	
+++ b/tests/baselines/reference/tsConfig/Initialized TSConfig with incorrect compiler option value/tsconfig.json	
@@ -88,7 +88,7 @@
     // "noImplicitReturns": true,                        /* Enable error reporting for codepaths that do not explicitly return in a function. */
     // "noFallthroughCasesInSwitch": true,               /* Enable error reporting for fallthrough cases in switch statements. */
     // "noUncheckedIndexedAccess": true,                 /* Include 'undefined' in index signature results */
-    // "noImplicitOverride": true,                       /* Add `undefined` to a type when accessed using an index. */
+    // "noImplicitOverride": true,                       /* Ensure overriding members in derived classes are marked with an override modifier. */
     // "noPropertyAccessFromIndexSignature": true,       /* Enforces using indexed accessors for keys declared using an indexed type */
     // "allowUnusedLabels": true,                        /* Disable error reporting for unused labels. */
     // "allowUnreachableCode": true,                     /* Disable error reporting for unreachable code. */
diff --git a/tests/baselines/reference/tsConfig/Initialized TSConfig with incorrect compiler option/tsconfig.json b/tests/baselines/reference/tsConfig/Initialized TSConfig with incorrect compiler option/tsconfig.json
index fabb4d8bc80..a936136ad6a 100644
--- a/tests/baselines/reference/tsConfig/Initialized TSConfig with incorrect compiler option/tsconfig.json	
+++ b/tests/baselines/reference/tsConfig/Initialized TSConfig with incorrect compiler option/tsconfig.json	
@@ -88,7 +88,7 @@
     // "noImplicitReturns": true,                        /* Enable error reporting for codepaths that do not explicitly return in a function. */
     // "noFallthroughCasesInSwitch": true,               /* Enable error reporting for fallthrough cases in switch statements. */
     // "noUncheckedIndexedAccess": true,                 /* Include 'undefined' in index signature results */
-    // "noImplicitOverride": true,                       /* Add `undefined` to a type when accessed using an index. */
+    // "noImplicitOverride": true,                       /* Ensure overriding members in derived classes are marked with an override modifier. */
     // "noPropertyAccessFromIndexSignature": true,       /* Enforces using indexed accessors for keys declared using an indexed type */
     // "allowUnusedLabels": true,                        /* Disable error reporting for unused labels. */
     // "allowUnreachableCode": true,                     /* Disable error reporting for unreachable code. */
diff --git a/tests/baselines/reference/tsConfig/Initialized TSConfig with list compiler options with enum value/tsconfig.json b/tests/baselines/reference/tsConfig/Initialized TSConfig with list compiler options with enum value/tsconfig.json
index d71d6cd9dfe..21798af11aa 100644
--- a/tests/baselines/reference/tsConfig/Initialized TSConfig with list compiler options with enum value/tsconfig.json	
+++ b/tests/baselines/reference/tsConfig/Initialized TSConfig with list compiler options with enum value/tsconfig.json	
@@ -88,7 +88,7 @@
     // "noImplicitReturns": true,                        /* Enable error reporting for codepaths that do not explicitly return in a function. */
     // "noFallthroughCasesInSwitch": true,               /* Enable error reporting for fallthrough cases in switch statements. */
     // "noUncheckedIndexedAccess": true,                 /* Include 'undefined' in index signature results */
-    // "noImplicitOverride": true,                       /* Add `undefined` to a type when accessed using an index. */
+    // "noImplicitOverride": true,                       /* Ensure overriding members in derived classes are marked with an override modifier. */
     // "noPropertyAccessFromIndexSignature": true,       /* Enforces using indexed accessors for keys declared using an indexed type */
     // "allowUnusedLabels": true,                        /* Disable error reporting for unused labels. */
     // "allowUnreachableCode": true,                     /* Disable error reporting for unreachable code. */
diff --git a/tests/baselines/reference/tsConfig/Initialized TSConfig with list compiler options/tsconfig.json b/tests/baselines/reference/tsConfig/Initialized TSConfig with list compiler options/tsconfig.json
index 067fd945939..f9f3d275cf3 100644
--- a/tests/baselines/reference/tsConfig/Initialized TSConfig with list compiler options/tsconfig.json	
+++ b/tests/baselines/reference/tsConfig/Initialized TSConfig with list compiler options/tsconfig.json	
@@ -88,7 +88,7 @@
     // "noImplicitReturns": true,                        /* Enable error reporting for codepaths that do not explicitly return in a function. */
     // "noFallthroughCasesInSwitch": true,               /* Enable error reporting for fallthrough cases in switch statements. */
     // "noUncheckedIndexedAccess": true,                 /* Include 'undefined' in index signature results */
-    // "noImplicitOverride": true,                       /* Add `undefined` to a type when accessed using an index. */
+    // "noImplicitOverride": true,                       /* Ensure overriding members in derived classes are marked with an override modifier. */
     // "noPropertyAccessFromIndexSignature": true,       /* Enforces using indexed accessors for keys declared using an indexed type */
     // "allowUnusedLabels": true,                        /* Disable error reporting for unused labels. */
     // "allowUnreachableCode": true,                     /* Disable error reporting for unreachable code. */
diff --git a/tests/baselines/reference/tscWatch/programUpdates/should-not-trigger-recompilation-because-of-program-emit/declarationDir-is-specified.js b/tests/baselines/reference/tscWatch/programUpdates/should-not-trigger-recompilation-because-of-program-emit/declarationDir-is-specified.js
index 079ef7d3972..ffde45ed51c 100644
--- a/tests/baselines/reference/tscWatch/programUpdates/should-not-trigger-recompilation-because-of-program-emit/declarationDir-is-specified.js
+++ b/tests/baselines/reference/tscWatch/programUpdates/should-not-trigger-recompilation-because-of-program-emit/declarationDir-is-specified.js
@@ -109,7 +109,7 @@ interface Array<T> { length: number; [n: number]: T; }
     // "noImplicitReturns": true,                        /* Enable error reporting for codepaths that do not explicitly return in a function. */
     // "noFallthroughCasesInSwitch": true,               /* Enable error reporting for fallthrough cases in switch statements. */
     // "noUncheckedIndexedAccess": true,                 /* Include 'undefined' in index signature results */
-    // "noImplicitOverride": true,                       /* Add `undefined` to a type when accessed using an index. */
+    // "noImplicitOverride": true,                       /* Ensure overriding members in derived classes are marked with an override modifier. */
     // "noPropertyAccessFromIndexSignature": true,       /* Enforces using indexed accessors for keys declared using an indexed type */
     // "allowUnusedLabels": true,                        /* Disable error reporting for unused labels. */
     // "allowUnreachableCode": true,                     /* Disable error reporting for unreachable code. */
diff --git a/tests/baselines/reference/tscWatch/programUpdates/should-not-trigger-recompilation-because-of-program-emit/when-outDir-and-declarationDir-is-specified.js b/tests/baselines/reference/tscWatch/programUpdates/should-not-trigger-recompilation-because-of-program-emit/when-outDir-and-declarationDir-is-specified.js
index 491c87efcc4..7d8c7dc00f4 100644
--- a/tests/baselines/reference/tscWatch/programUpdates/should-not-trigger-recompilation-because-of-program-emit/when-outDir-and-declarationDir-is-specified.js
+++ b/tests/baselines/reference/tscWatch/programUpdates/should-not-trigger-recompilation-because-of-program-emit/when-outDir-and-declarationDir-is-specified.js
@@ -109,7 +109,7 @@ interface Array<T> { length: number; [n: number]: T; }
     // "noImplicitReturns": true,                        /* Enable error reporting for codepaths that do not explicitly return in a function. */
     // "noFallthroughCasesInSwitch": true,               /* Enable error reporting for fallthrough cases in switch statements. */
     // "noUncheckedIndexedAccess": true,                 /* Include 'undefined' in index signature results */
-    // "noImplicitOverride": true,                       /* Add `undefined` to a type when accessed using an index. */
+    // "noImplicitOverride": true,                       /* Ensure overriding members in derived classes are marked with an override modifier. */
     // "noPropertyAccessFromIndexSignature": true,       /* Enforces using indexed accessors for keys declared using an indexed type */
     // "allowUnusedLabels": true,                        /* Disable error reporting for unused labels. */
     // "allowUnreachableCode": true,                     /* Disable error reporting for unreachable code. */
diff --git a/tests/baselines/reference/tscWatch/programUpdates/should-not-trigger-recompilation-because-of-program-emit/when-outDir-is-specified.js b/tests/baselines/reference/tscWatch/programUpdates/should-not-trigger-recompilation-because-of-program-emit/when-outDir-is-specified.js
index 00b39a1134c..2339452e2ba 100644
--- a/tests/baselines/reference/tscWatch/programUpdates/should-not-trigger-recompilation-because-of-program-emit/when-outDir-is-specified.js
+++ b/tests/baselines/reference/tscWatch/programUpdates/should-not-trigger-recompilation-because-of-program-emit/when-outDir-is-specified.js
@@ -109,7 +109,7 @@ interface Array<T> { length: number; [n: number]: T; }
     // "noImplicitReturns": true,                        /* Enable error reporting for codepaths that do not explicitly return in a function. */
     // "noFallthroughCasesInSwitch": true,               /* Enable error reporting for fallthrough cases in switch statements. */
     // "noUncheckedIndexedAccess": true,                 /* Include 'undefined' in index signature results */
-    // "noImplicitOverride": true,                       /* Add `undefined` to a type when accessed using an index. */
+    // "noImplicitOverride": true,                       /* Ensure overriding members in derived classes are marked with an override modifier. */
     // "noPropertyAccessFromIndexSignature": true,       /* Enforces using indexed accessors for keys declared using an indexed type */
     // "allowUnusedLabels": true,                        /* Disable error reporting for unused labels. */
     // "allowUnreachableCode": true,                     /* Disable error reporting for unreachable code. */
diff --git a/tests/baselines/reference/tscWatch/programUpdates/should-not-trigger-recompilation-because-of-program-emit/with-outFile.js b/tests/baselines/reference/tscWatch/programUpdates/should-not-trigger-recompilation-because-of-program-emit/with-outFile.js
index 09b2aa2f335..b28214cfa6f 100644
--- a/tests/baselines/reference/tscWatch/programUpdates/should-not-trigger-recompilation-because-of-program-emit/with-outFile.js
+++ b/tests/baselines/reference/tscWatch/programUpdates/should-not-trigger-recompilation-because-of-program-emit/with-outFile.js
@@ -109,7 +109,7 @@ interface Array<T> { length: number; [n: number]: T; }
     // "noImplicitReturns": true,                        /* Enable error reporting for codepaths that do not explicitly return in a function. */
     // "noFallthroughCasesInSwitch": true,               /* Enable error reporting for fallthrough cases in switch statements. */
     // "noUncheckedIndexedAccess": true,                 /* Include 'undefined' in index signature results */
-    // "noImplicitOverride": true,                       /* Add `undefined` to a type when accessed using an index. */
+    // "noImplicitOverride": true,                       /* Ensure overriding members in derived classes are marked with an override modifier. */
     // "noPropertyAccessFromIndexSignature": true,       /* Enforces using indexed accessors for keys declared using an indexed type */
     // "allowUnusedLabels": true,                        /* Disable error reporting for unused labels. */
     // "allowUnreachableCode": true,                     /* Disable error reporting for unreachable code. */
diff --git a/tests/baselines/reference/tscWatch/programUpdates/should-not-trigger-recompilation-because-of-program-emit/without-outDir-or-outFile-is-specified-with-declaration-enabled.js b/tests/baselines/reference/tscWatch/programUpdates/should-not-trigger-recompilation-because-of-program-emit/without-outDir-or-outFile-is-specified-with-declaration-enabled.js
index 56d6eed8b89..9c9e53d029c 100644
--- a/tests/baselines/reference/tscWatch/programUpdates/should-not-trigger-recompilation-because-of-program-emit/without-outDir-or-outFile-is-specified-with-declaration-enabled.js
+++ b/tests/baselines/reference/tscWatch/programUpdates/should-not-trigger-recompilation-because-of-program-emit/without-outDir-or-outFile-is-specified-with-declaration-enabled.js
@@ -109,7 +109,7 @@ interface Array<T> { length: number; [n: number]: T; }
     // "noImplicitReturns": true,                        /* Enable error reporting for codepaths that do not explicitly return in a function. */
     // "noFallthroughCasesInSwitch": true,               /* Enable error reporting for fallthrough cases in switch statements. */
     // "noUncheckedIndexedAccess": true,                 /* Include 'undefined' in index signature results */
-    // "noImplicitOverride": true,                       /* Add `undefined` to a type when accessed using an index. */
+    // "noImplicitOverride": true,                       /* Ensure overriding members in derived classes are marked with an override modifier. */
     // "noPropertyAccessFromIndexSignature": true,       /* Enforces using indexed accessors for keys declared using an indexed type */
     // "allowUnusedLabels": true,                        /* Disable error reporting for unused labels. */
     // "allowUnreachableCode": true,                     /* Disable error reporting for unreachable code. */
diff --git a/tests/baselines/reference/tscWatch/programUpdates/should-not-trigger-recompilation-because-of-program-emit/without-outDir-or-outFile-is-specified.js b/tests/baselines/reference/tscWatch/programUpdates/should-not-trigger-recompilation-because-of-program-emit/without-outDir-or-outFile-is-specified.js
index 6f826876df7..0415e06b6f3 100644
--- a/tests/baselines/reference/tscWatch/programUpdates/should-not-trigger-recompilation-because-of-program-emit/without-outDir-or-outFile-is-specified.js
+++ b/tests/baselines/reference/tscWatch/programUpdates/should-not-trigger-recompilation-because-of-program-emit/without-outDir-or-outFile-is-specified.js
@@ -109,7 +109,7 @@ interface Array<T> { length: number; [n: number]: T; }
     // "noImplicitReturns": true,                        /* Enable error reporting for codepaths that do not explicitly return in a function. */
     // "noFallthroughCasesInSwitch": true,               /* Enable error reporting for fallthrough cases in switch statements. */
     // "noUncheckedIndexedAccess": true,                 /* Include 'undefined' in index signature results */
-    // "noImplicitOverride": true,                       /* Add `undefined` to a type when accessed using an index. */
+    // "noImplicitOverride": true,                       /* Ensure overriding members in derived classes are marked with an override modifier. */
     // "noPropertyAccessFromIndexSignature": true,       /* Enforces using indexed accessors for keys declared using an indexed type */
     // "allowUnusedLabels": true,                        /* Disable error reporting for unused labels. */
     // "allowUnreachableCode": true,                     /* Disable error reporting for unreachable code. */