Normal vector

A vector pointing straight off a plane (or surface), perpendicular to everything lying in it.

A normal vector to a plane is any vector pointing straight off it, perpendicular to every vector lying in the plane. It is never unique — any nonzero scalar multiple of $\mathbf{n}$ describes the same plane — but the coefficients $a,b,c$ in $ax+by+cz=d$ literally are a normal vector's components, so a normal can be read off a plane's equation directly. The cross product of any two vectors lying in a plane is automatically a normal vector to it.

Examples

For the plane $2x-y+3z=6$ , a normal vector is $\mathbf{n}=\langle 2,-1,3\rangle$ .
A renderer needs a surface's outward-facing normal to shade it correctly.