Windows RT is the official name of ARM-based Windows 8

Communications manager, Brandon LeBlanc, took to Microsoft’s official blog to set the record straight regarding the release versions of Windows 8. First and foremost, Windows 8 will be the official name for x86/64 editions of Windows. Windows 8 will have 2 editions: Windows 8 and Windows 8 Pro.
On the mobile side, Windows RT will be the official name for Windows on Arm (also previously referred to as WOA). Windows RT will only be available pre-installed and will include touch-optimized desktop versions of the new Microsoft Word, Excel, PowerPoint, and OneNote. Windows RT will utilize the WinRT runtime to support cloud-enabled, touch-enabled, web-connected apps.
source: Microsoft