Rew
A rew, or worlddisk, is a mound shaped like a flat disk. In some universes, all or most worlds are rews, Charos being perhaps one of the most paradigmatic examples. In other, more stromatic universes, rews may exist among diverse worlds of other shapes; in still other planes, a rew may exist as an isolated world alone in a void.
Technically, a rew is a subclass of aphedon, but one common enough to have its own name.
Etymology
The word "rew" derives from the Middle French roue, referring to a wheel or anything of similar shape—this word survives in modern French with essentially the same meaning. Roue in turn has its origin in the Latin rota, which in turn can be traced back to the Proto-Indo-European root *Hret-, to run or roll. Other English words descended from the same Latin lexon and therefore distantly related to "rew" include "rotate", "roll", "rotund", and "roulette".
The spelling of "rew" may have been influenced by rewel or rewell, Middle English spellings of the word now usually spelled rowel, which in modern English has retained only specialized meanings related to heraldry or horse tack but which formerly, like the French roue, had a broader range of meanings encompassing most anything wheel-shaped. In fact, it was once commonly thought that rew came from an apocope of rewel, perhaps motivated by the fact that the ending originated from the diminutive suffix -el and hence was inapplicable to the decidedly undiminutive worlddisks. However, this is now no longer believed to be the case—at least on most worlds, though it is not impossible that the word did have this origin on some worlds through convergent etymology.
There is no etymological connection with the now dialectal homograph rew that is a synonym (or merely an alternate spelling) of row. This word derives ultimately from an entirely different Proto-Indo-European root.
Shape
By defition, a rew is flat, thin, and circular. It need not, of course, be, and generally is not, perfectly circular; a roughly circular world with some irregularity at the edge may still be considered a rew, as may a flat elliptical world of low eccentricity, but a world that deviates too far from a circular shape generally will not. A world shaped like too thick a cylinder also will not usually be considered a rew; rews generally have a thickness only a small fraction of their radius. There are no hard-and-fast criteria for just how much a world's shape can differ from a perfect circle or precisely how close its thickness can be to its radius before it's no longer a rew, but in practice there are relatively few worlds with a shape close enough to a thin disk to possibly qualify as a rew but far enough for that categorization to be questionable. In some cases, worlds of very different shapes can still be considered rews if they seem to be derived from or closely linked to circular worlds. In Charos, for example, there are rare and unusual worlds of highly irregular shapes, including the still mysterious brotoids, which are nevertheless counted among rews because they seem to be variants of the much more numerous Charotian worlds that are circular in shape.
Rews differ in cross-sectional profile, though those of the same universe tend to be of similar form. Some rews, including those of Charos, are of close to uniform thickness, with flat edges and rectangular cross-section. Others taper toward the edges, lenticular or elliptical in cross-sectional shape; yet others are thicker at the edges and biconcave, each side of the rew a shallow bowl. Concavo-convex or convexo-concave rews are also possible, though rarer, as are rews of more complicated cross-sections, but of course in any case the change in thickness must be quite gradual in order for the world to be flat enough to be considered a rew.
Gravity
The strength and direction of gravity on the surface of a rew depends on the physics of the dition in which it lies. Most commonly, it acts perpendicular to the rews' faces. In some cases it acts in the same direction on both sides of the rew, which means that it accelerates objects toward one side and away from the other. If this holds, of course, then only the side where gravity acts toward the surface can have objects rest on it and is generally inhabitable; this side is effectively the "top" of the rew and the other side, or underside, is the bottom. In other cases gravity acts toward the surface of both sides. When this is true, then both sides may be habitable, and each side is considered the obverse of the other.
Other possibilities have been encountered, but are substantially rarer. There are rews where gravity points directly toward the rew's axis of symmetry; on such worlds, any inhabitation is generally confined to the rew's outer rim. On others, gravity points directly toward the rew's center; there, near the center of the rew's surface, conditions are similar to those on a world where gravity is perpendicular, but the farther one ventures from the center the greater the "horizontal" component of the gravitational force, and moving in a direction away from the center of the rew is effectively tantamount to moving uphill. Rews with either of these gravitational conditions tend to be significantly thicker than other rews in comparison with their radii, in some cases the two dimensions being of the same order of magnitude, though the thickness is still smaller (if it weren't, few would consider the world a rew).
If there is an underfold or other hollows in the rew's interior, gravity there is usually consistent with that on its exterior. If the gravitational orientation is the same on both sides of the rew, it is likely to be the same inside it as well; if the gravitational orientation is opposite on the rew's two surfaces, then inside it usually transitions from one direction to another, and the gravitational field vanishes entirely in the rew's central plane.
The rews of Charos are a special case in that in the esture of Ses gravity is not an external force, but one intrinsic to the objects it acts on, so the direction of gravity depends not on the rew itself but on the objects on its surface. Still, because it is objects with their weight vectors pointing toward the rew's surface that are best able to remain and act on that surface, for most practical purposes the rews of Charos functionally have gravity pointing toward both surfaces of the disk. In hollow spaces inside the rews, however, objects of any gravitational orientation may exist, since their enclosed space prevents them from falling into space—although the vast majority of objects have the same orientation as those on one of the rew's surfaces, if only because objects of other orientations are unlikely to find their way into the rews.
Formation
Rews may occur naturally under certain physical and stœcheiotic conditions. In an esture like Wicik where gravity points toward the center of mass and obeys an inverse square law and is the strongest influence on a world's formation, a disk-shaped world would collapse under its own weight; there is a reason why the planets of Herit are spheres. (A disk spinning fast enough might escape this fate, but would have the opposite problem, in that gravity would be unlikely to provide enough centripetal force to hold it together.) However, different circumstances may allow the formation of rews, such as gravitational force oriented mostly toward certain planes, or magical resonances favoring the formation of circular shapes.
Rews may, of course, also be artificially created. In that case, they may hold their shape in defiance of physical laws through magical or technological means, perhaps being located within a rhegus imposing a local dition or perhaps being fashioned of some unusually strong and rigid materials that are able to withstand the forces that would impose on it a different shape.