Where does the name Nine Worlds come from?

Well, in the beginning, there were two regions: Muspellsheim in the south, full of fire, light and heat; and Niflheim in the north, full of arctic waters, mists, and cold. Between them stretched the yawning emptiness of Ginnungagap, and into it poured sparks and smoke from the south and layers of rime-ice and glacial rivers from the north. As heat and cold met in Ginnungagap, a living Jötunn, Ymir, appeared in the melting ice. From his left armpit, the first man and woman were born. From his legs, the frost jötnar were born, making Ymir the progenitor of the jötnar.*

So anyways, yada yada, some more stuff happened, and now here we are running an annual event that seeks to bring together an entire constellation of different people and fandoms and nestle them in the branches of our world tree. SO IT'S THE OBVIOUS NAME FOR US TO TAKE.

* - thanks, Wikipedia!