A planet that lacks ground, or has been settled high in the clouds. You're so far up, and the sky is so cloudy, you rarely see the surface. Often, this is a Gas Giant that luckily has Oxygen and Nitrogen as it's main gases. In other depictions, it's a Terrestrial Planet that has floating cities for no good reason. In that case, there may be some sort of Class Struggle where the poor live on the surface, and the rich float in the clouds.