Midsphere
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A sphere containing on its surface one point from each edge of a semiregular or regular polyhedron. It is also called an intersphere, reciprocating sphere, and inversion sphere. The radius of this sphere is called the midradius.
Source: Eric W. Weisstein. "Midsphere." From MathWorld--A Wolfram Web Resource. [1]
See also: Inscribed sphere, Circumscribed sphere