Prime Material Plane
The Prime Material Plane is the plane where the world of Ardrana exists -- the "home plane", if you will, of all its adventurers. However, that is not the only world that exists on the Prime Material. The Prime Material is just one plane, but it contains lots of individual worlds. A world may be only a single planet or it may be a complete system with planets, moons, asteroids, comets, stars, and more. Each world is sealed like a bubble in its own crystal sphere, and that sphere is suspended within an amber stream called the phlogiston. (Those that know can travel from sphere to sphere through the phlogiston by a process known as spelljamming, but that's neither here nor there.)
Some of the better-known worlds on the Prime Material Plane are Toril with its Tears of Selune and Krynn with its three moons and vanishing stars. Those and all the others are unique places with vastly different cultures and celestial topographies, but they're all plainly called "worlds," just the same. Indeed, any world that isn't planar can be found somewhere on the Prime Material Plane, provided you know where to look. The worlds and occupants of the Prime Material contrast based on the current development of culture, science, magic, and natural evolution.
Note: Some of this information on the Prime Material Plane originally appeared in the Planescape AD&D campaign setting from TSR. Its use here is for the purposes of providing context for the campaign only.