The plane through the midpoint of a line segment and perpendicular to that segment, also called a mediating plane. The term "mediator" was introduced by J. Neuberg (Altshiller-Court 1979, p. 298).
Mediator
See also
Midpoint, PlaneExplore with Wolfram|Alpha
References
Altshiller-Court, N. Modern Pure Solid Geometry. New York: Chelsea, p. 1, 1979.Referenced on Wolfram|Alpha
MediatorCite this as:
Weisstein, Eric W. "Mediator." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/Mediator.html