Definitions
1.NounA middle sphere or region.
2.NounThe sphere which, if it exists, is tangent to every edge of a given polyhedron and with respect to which the polyhedron's vertices are the inversion poles of the planes of the faces of its dual polyhedron (and vice versa).
3.NounThe sphere, between two given spheres, by which each of the latter may be inverted into the other.
4.NounA supposed realm between earth and heaven.