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*.