Six Classes of Beings

In Buddhist cosmology, one world system comprises of a flat disc with a central mountain encircled by an ocean in which there are four major continents. This is orbited by the sun and moon. One thousand such worlds make "a thousand world-system". A thousand of these grouped together, make "a million world-system". A thousand of those make up "the billion world-system" (literally the "three-thousands") which is our universe, and the field of activity of our successive Buddhas such as Shakyamuni, a thousand million Buddhas appearing simultaneously, one in each world. [Peter Roberts]