"Spelljammer is a campaign setting for the Advanced Dungeons & Dragons (2nd edition) role-playing game, which features a fantastical (as opposed to scientific) outer space environment.
Spelljammer introduced into the AD&D universe a comprehensive system of fantasy astrophysics, including the Ptolemaic concept of crystal spheres. Crystal spheres may contain multiple worlds and are navigable using ships equipped with "spelljamming helms". Ships powered by spelljamming helms are capable of flying into not only the sky but into space. With their own fields of gravity and atmosphere, the ships have open decks and tend not to resemble the spaceships of science fiction, but instead look more like galleons, animals, birds, fish or even more wildly fantastic shapes.
The Spelljammer setting is designed to allow the usual sword and sorcery adventures of Dungeons & Dragons to take place within the framework of outer space tropes. Flying ships travel through the vast expanses of interplanetary space, visiting moons and planets and other stellar objects.
Like the Planescape setting, Spelljammer unifies most of the other AD&D settings and provides a canonical method for allowing characters from one setting (say, Dragonlance) to travel to another (say, the Forgotten Realms). However, unlike Planescape it keeps all of the action on the Prime Material Plane and uses the crystal spheres, and the "phlogiston" (a bright, extremely combustible gas-like medium) between them, to form natural barriers between otherwise incompatible settings. Though the cosmology is derived largely from the Ptolemaic system of astronomy, many of the ideas owe much to the works of Jules Verne and his contemporaries, and to related games and fiction with a steampunk or planetary romance flavor. A strong Age of Sail flavor is also present."
Source: Wikipedia, "Spelljammer", available under the CC-BY-SA License.