It was a slip of the tongue! The truth is that Nirn was Earth 1.2 Billion years ago!

Tamriel = A Continent.
Akivir = Another Continent.
Nirn = The planet.
Secunda = A moon
Masser = The other moon
Mundus = The material plane
Oblivion = Hell, basically. However, there are many different "pockets" of Oblivion; not all of them "Hellish."
Aetherius/Sovngarde = Heaven; the plane of Immortals (which is funny, because it's where the Aedra come from, and the Aedra are not immortal XD)