diff --git a/MdePkg/Include/IndustryStandard/AcpiAml.h b/MdePkg/Include/IndustryStandard/AcpiAml.h
index 6919622af2..3dd5408b35 100644
--- a/MdePkg/Include/IndustryStandard/AcpiAml.h
+++ b/MdePkg/Include/IndustryStandard/AcpiAml.h
@@ -2,6 +2,7 @@
   This file contains AML code definition in the latest ACPI spec.
 
   Copyright (c) 2011, Intel Corporation. All rights reserved.<BR>
+  Copyright (c) 2019, ARM Limited. All rights reserved.<BR>
   SPDX-License-Identifier: BSD-2-Clause-Patent
 
 **/
@@ -30,6 +31,7 @@
 #define AML_PACKAGE_OP               0x12
 #define AML_VAR_PACKAGE_OP           0x13
 #define AML_METHOD_OP                0x14
+#define AML_EXTERNAL_OP              0x15
 #define AML_DUAL_NAME_PREFIX         0x2e
 #define AML_MULTI_NAME_PREFIX        0x2f
 #define AML_NAME_CHAR_A              0x41