The Phlogiston, also known as the Rainbow Ocean and the Flow is a Transitive Plane of Dungeons & Dragons that is coterminous with the Prime Material... at least, that's one way of looking at it. Another way is to look at it as being the "backbone" of the Prime Material, with the various worlds of the Prime existing inside "bubbles" of faux-outer space, known as Wildspace, within the Phlogiston, which are called Crystal Spheres (or Crystal Shells, in some sources). It is one of the two key setting elements of Spelljammer.
As its nickname suggests, the Phlogiston resembles an infinite 3d expanse of rainbow colored, faintly luminescent, breathable mist. It's quite possible to see in the phlogiston, but difficult, as the swirling vapors cloud the eyes in much the same way as wandering through a bank of heavy fog. Oh, and using light is very much a bad idea; the phlogiston is flammable and so doesn't mix well with fire. Even a spark can set off a small, fiery explosion. If a spell is cast that produces flame, it goes off instantly, centered on the caster, and it is considerably stronger than normal - for obvious reasons, evokers and other blasty casters traveling in the phlogiston are advised to switch to spells that stem from lightning, thunder, acid, frost... really, any element other than fire!
The Flow's vastness is dominated by flowrivers, which link crystal shells in a mind-bogglingly gigantic 3-D web. Flowrivers are useful to spacefarers because they (greatly) facilitate travel between the crystal shells. In many cases, a journey directly to a specific crystal shell would take many years, but the journey can easily be shortened to a much smaller amount of time by following the flowrivers from one shell to the next (hence, navigation in the phlogiston is a tricky connect-the-dots activity).
It is not possible to see a flowriver (though detailed measurements can detect a significant increase in the drift of the multi-colored fog in one or two directions). They are felt more than seen; an individual sitting on a spelljammer helm can easily realize that a starship has entered a flowriver.
Most flowrivers are two-way, allowing passage in either direction between two crystal shells. However, some are one-way, so it is not advisable to try to double back along a one-way flowriver (doing so poses the same difficulties as not using a flowriver at all).
Most places within the phlogiston look exactly alike, but there are known, and dreaded, Dark Regions from which few have returned. A feature unique to the Rainbow Ocean is the strange flow beacon, an object that resembles a rapidly spinning star. No one has ever determined who built these devices, nor what they do.