I believe it's called Toroidal mapping. http://www.wildbunny.co.uk/blog/2012/04/13/implementing-a-wrap-around-world/
I did something similar by making the level out of 4 tilemaps which were slightly larger than the screen size then repositioning them as the camera scrolled to create the illusion on a wrapped map. The hard part was repositioning the objects so they moved with the tilemaps. To be honest the maths was a mess and the game slows down a lot with many objects so i think the method i used wasn't any good.
It would be cool to see how a maths guru would do a proper job of it, there are some very clever people on the forums.