From the Super Mario Wiki
Nimbus Land is a floating kingdom on a massive cloud where cloud-people live in Super Mario RPG: Legend of the Seven Stars. It is ruled by King Nimbus and Queen Nimbus, who have a son, Prince Mallow. Nimbus Land also refers to World 6 of the game, and includes the level Nimbus Land itself, as well as Barrel Volcano.
The giant cloud of Nimbus Land is attached to a vine accessible from Bean Valley, the only known way to access it from the ground. The palace of King Nimbus stands in the center of the town. Normally, it houses many statues and birds, of which the king is fond of both, and the citizens of Nimbus Land are free to enter. During Valentina's attempted usurpation, however, the palace is sealed to the public and its halls are filled with monsters.
Other noteworthy locations include the usual item shop and inn, Garro's studio, the docking point for the Royal Bus, and the royal hot springs. The Nimbus innkeeper gives the option of using his special "Dream Cushion", which costs an extra 30 coins and causes Mario to have (often bizarre) dreams. The hot springs instantly restore HP and FP and are situated directly over Barrel Volcano, but only royalty (and apparently royalty's party members) can bathe there.
For a long time, Prince Mallow was missing. Valentina, however, pretended she found the prince during a plot to become queen of Nimbus Land. 'Prince Mallow' was actually Dodo, a large bird, but the population of Nimbus Land was convinced for a while that he was the true prince. However, Mario discovered the plot. The real Mallow was traveling with him. They, along with the other members of their party, stop Valentina and Dodo and rescue the king and queen.
Later, Croco comes to Nimbus Land in his ongoing search for treasure. He makes no attempt to fight Mario this time, though in his haste he drops the Signal Ring that he would planned to use. Mario feels no guilt over taking the thief's forgotten prize. By the next time they meet, Croco seems to have forgotten he ever owned it.
Nimbus Land's name refers to the Latin word for cloud, nimbus.
 Enemies Located in Nimbus Land
 Names in Other Languages