Line that touches a curve at a single point without crossing it.
The limit of a secant to a curve as one of the points of intersection, P’, moves ever closer to the point P while remaining on the curve.
In the case of a circle, the point of contact P of the tangent to the circle is called the point of tangency.
The line that contains points P and P’ is a secant to the circle.
Example
The radius r that joins the centre of the circle to the point of tangency P with line t is perpendicular to t.