A D&D campaign setting that tied the different worlds in Prime Material together without the use of the material from Planescape.
Practically speaking Spelljammer was sword and sorcery in space.
The first video game about Spelljammer was released on January 1, 1992.
WotC owns the rights to it but have not pursued it in any way, other than giving permission for Beyond the Moons (spelljammer.org) to support it.
In addition to unique adventures in the SpellJammer settings, It provides a way to use characters in multiple D&D campaign settings that does not violate the canonical D&D fiction. SpellJammer is part of the same material plane as the other material plane settings. SpellJammer materials can be used in any of the other settings. For instance a spacecraft can fly near the ground, float, and even go underwater. Suitable creatures, such as Dragons, prevent this from being an gameplay imbalance. The physics of space travel in the D&D universe are based on fantasy instead of science (science-fiction). They are based on the [url=http://en.wikipedia.org/wiki/Geocentric_model#Ptolemaic_system]Ptolemaic system[/uvl] without nested spears and include fantastic rules for maintaining an exposed bubble of air in the vacuum of space and earth-like gravity on ships that always attracts objects in the most practical direction.