The common axis of the three altitude planes of a trihedron.
Orthocentric Line
See also
TrihedronExplore with Wolfram|Alpha
References
Altshiller-Court, N. "The Orthocentric Line." §2.1 in Modern Pure Solid Geometry. New York: Chelsea, pp. 27-30, 1979.Referenced on Wolfram|Alpha
Orthocentric LineCite this as:
Weisstein, Eric W. "Orthocentric Line." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/OrthocentricLine.html