RTT

From Ogre Wiki

RTT - In the context of Ogre, Render To Texture, a technique where the rendered pixels are blitted to a texture, instead of to the screen.

The otherwise most common usage of the acronym is Real-Time Tactics, a sub-genre of strategy games sibling to RTS (real-time strategy).

For the usage of this method in Ogre, have a look at the Intermediate Tutorial 7.