The Thousandfold Refraction of Rabiah was a mysterious event in which the plane Rabiah was reproduced one-thousand times, creating a thousand and one virtually identical worlds. While fully incorporated into prerevisionist storyline, many theorize that the concept of the Thousand-fold Refraction came about as a flavorful explanation for why there could be multiples of the various characters in play, before the Legend rule existed to handle such things.
The "thousand and one" part is a nod to the real Arabian Nights, originally known as One Thousand and One Nights.