Normal
Synonym for perpendicular line.
A tangent is perpendicular to a radius drawn to the point of tangency of a circle. The line that contains the radius is the normal to the circle at the point of tangency.
Example
For the point [latex]P\left ( x_{1}, y_{1} \right )[/latex] on the circle [latex]\left ( x-h \right )^{2}+\left ( y-k \right )^{2}=r^{2}[/latex], the slope of the normal is [latex]\frac{y-k}{x-h}[/latex] and its equation is[latex]\frac{\left ( y-y_{1} \right )}{\left ( x-x_{1} \right )}=\frac{\left ( h-y_{1} \right )}{\left ( k-x_{1} \right )}[/latex].