Terrestria is the plane on which the planet Earth is located. EDIT: Due to limited Magic information, the best assumption is that planes are individual planets. No other information can be garnered from the true Magic: the Gathering storyline. The best explanation for our universe being in the MtG multiverse is billions of overlays on a gigantic plane, causing each individual cosmic body that exists in real life. EDIT: The formation of a seperate article, i.e. 'The AoM Multiverse' is suggested.