Noun
A middle sphere or region.
(geometry, solid geometry) The 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).
(geometry) The sphere, between two given spheres, by which each of the latter may be inverted into the other.
(religion, Vedic mythology, uncountable) A supposed realm between earth and heaven.
Source: en.wiktionary.org