"I never get tired of looking at that when I come home."
-- frequent sentiment of many individuals returning to a home planet
Definition
A "planet" in the Spelljammer setting is a large celestial body that (generally) fits all of the following criteria, though there are exceptions:
Is large enough to possess an independent gravity field (standard rules do not apply; the gravity fields of stellar objects are almost always extremely small for the size of the object)
Does not orbit another stellar body which orbits a primary
Is not a strange object that forms a tail when its extremely oval orbit brings it close to a primary (that is, not a comet)
A planet usually meet the following secondary criteria, as well:
Orbits a primary in such a manner that the orbit is circular, or nearly so
Is shaped like a sphere, a cube, or other recognizable shape, including irregular
Is at least 500 miles in diameter; smaller objects are usually called asteroids (but smaller objects might be called planets, according to local traditions, and this criterion definitely does not apply to cluster worlds)
Is not located inside an asteroid belt (exceptions exist; these are almost always dwarf planets)
Notes
It is possible for a planet to be a primary (the planet Oerth in Greyspace is a very good example). In that event, it is still a planet
As noted above, cluster worlds are planets. Although the individual bodies of the cluster are typically far too small to be planets, the entire cluster qualifies as a planet
Typical Qualities
Most planets have atmospheres, but some are void worlds, with no air of any kind. The air is usually breathable, but again, not always
Some planets have smaller worlds that orbit them, called moons
Some planets also have rings around them
In rare cases, a moon may have a moon or rings, or even both
Let the space traveler beware
Some cultures may call something a planet that, objectively, is most certainly not; it is usually best to respect, or at least ignore and refrain from commenting upon, local customs!
See Also
Asteroid belt
Comet
Star
Orbit