A line along a normal vector (i.e., perpendicular to some tangent line).
If 
is a centrosymmetric
set which has a twice differentiable boundary, then there are 
normals through the center (Croft et al. 1991, p. 15).
Normal Line
See also
Double Normal, Normal Vector, Tangent LineExplore with Wolfram|Alpha
References
Croft, H. T.; Falconer, K. J.; and Guy, R. K. Unsolved Problems in Geometry. New York: Springer-Verlag, 1991.Referenced on Wolfram|Alpha
Normal LineCite this as:
Weisstein, Eric W. "Normal Line." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/NormalLine.html