Definition - What does Windows RT mean?
Windows RT is the official name for the Windows 8 OS version that runs on low-powered ARM processors. ARM processors are increasingly popular because they power many tablets and phones, as well as embedded and portable devices that rely on battery life.
When it was still in development, Windows RT was referred to as Windows on ARM.
Techopedia explains Windows RT
The "RT" stands for "runtime", for the Windows Runtime Library, the programming model for applications in the Metro design language. This is an intermediate level software library that allows developers to write an application once and be able to compile it in different ways, so that it runs on both Intel-type and ARM processors. This allows software programs and applications that are built using the WinRT APIs to be distributed for use on the 32-bit and 64-bit x86 Intel-based devices, like your office PC, as well as ARM devices like Windows Phone.