The normal section of a regular surface 
in the direction of a unit tangent
vector 
is the intersection of 
with the plane 
determined by 
and the surface normal 
.
Normal Section
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References
Gray, A. Modern Differential Geometry of Curves and Surfaces with Mathematica, 2nd ed. Boca Raton, FL: CRC Press, p. 365, 1997.Referenced on Wolfram|Alpha
Normal SectionCite this as:
Weisstein, Eric W. "Normal Section." From MathWorld--A Wolfram Resource. https://mathworld.wolfram.com/NormalSection.html