A tangent line is a line that
intersects a
circle at one point. Such a
line is said
to be tangent to that circle. The point at which the circle and the line
intersect is the point of tangency.

In the figure above, the line l is tangent to the circle C. Point T is
the point of tangency.

When a radius of a circle is drawn to a point of tangency (from the center,
of the circle, of course), that radius is
perpendicular to the tangent line
containing
that point of tangency. This means that for any tangent line, there exists a
perpendicular radius.

A tangent segment is a segment
with one
endpoint at the point of tangency and its other endpoint somewhere on the
tangent line. A tangent segment is also perpendicular to the radius of the
circle whose endpoint is the point of tangency.

Secant Lines

A secant line is a line that intersects a circle at two points. Every
secant line, therefore, contains a chord of the circle it intersects.