The sphere with respect to which inverse points are computed (i.e., with respect to which geometrical inversion is performed). For example, the cyclides are inversions in a sphere of tori. The center of the inversion sphere is called the inversion center, and its radius is called the inversion radius. When dual polyhedra are being considered, the inversion sphere is commonly called the midsphere (or intersphere, or reciprocating sphere).
In two dimensions, the inversion sphere collapses to an inversion circle.
