Windows on Windows, commonly referred to as WOW or WoW, is a name of two abstraction subsystems of Microsoft Windows operating system. WoW is a name of two critical components for backwards compatibility of legacy code as Windows progresses. The first component called WoW appeared in Windows NT to support applications using the Win16 API and was relying on NTVDM.
Supporting Win32 applications on Windows XP Professional x64 Edition, Windows Server 2003 x64 and other x64 versions of Windows requires a new WoW called WoW64. The x64 versions of Windows no longer include the 16 bit WoW subsystem (as well as the NTVDM) and can therefore no longer run 16-bit Windows nor DOS applications.