For 10 points, name this vector, which is perpendicular to a surface.
The flux of a vector field across a surface is given by the double integral, over the surface, of the vector
 field dotted with this quantity. The direction of this vector determines the orientation of a surface. For a
 plane, it can be found by taking the cross product of any two vectors in the plane; for a curve in space, it is
 equal to the derivative of the tangent vector. The direction of this vector, along with any point in the plane,
 defines the plane. For 10 points, name this vector, which is perpendicular to a surface.
The flux of a vector field across a surface is given by the double integral, over the surface, of the vector
 field dotted with this quantity.
the {normal} vector [or {N}; or {unit normal} vector]