Win64
From RemObjects Wiki
This is a Technical Glossary topic
Win64 is the 64bits of version of Windows, several versions exist of it, including Windows XP64, Windows 2003/64, Windows Vista 64 and Windows 2008. Windows 64 supports both 32 bits and 64 bits applications, though 16bits support has been dropped in this version.
Win64 has a WoW64 layer to support 32 bits applications under 64 bits Windows.