A unit normal vector is a normal vector of unit length. It is commonly denoted 
for an oriented surface. Reversing
the orientation replaces 
by 
.
For a regular space curve with nonzero curvature, the principal unit normal vector is commonly denoted 
. It points in the direction in which the unit
tangent vector changes and is defined by
 |
where 
is arc length. Together with the unit tangent 
and unit binormal
vector 
,
it forms the Frenet frame described by the Frenet
formulas.
