Definition- A secant is a line that intersects a circle at exactly two points. (Every secant contains a chord of a circle)

Definition- A tangent is a line that intersects a circle ate exactly one point. This point is called the point of tangency.

Postulate- A tangent line is perpendicular to the radius drawn to the point of tangency.

Postulate- if a line is perpendicular to a radius at its outer endpoint, then it is the tangent of the circle.

Definition- A tangent segment is that part of a tangent line between the point of contact and a point outside the circle.

Definition- A secant segment is the part of a secant line that joins a point outside the circle to the further intersection point of the secant and the circle.

Definition- A external part of a secant segment is the part of a secant line that joins the outside point to the closer intersection point.

Theorem- If two tangent segments are drawn to a circle from an external point those segments are congruent.

Definition- Tangent circles are two circles that intersect each other at exactly one point.

Definition- Externally tangent circles are tangent circles such that one circles lies outside the other.

Definition- Internally tangent circles are two tangent circles such that one circle lies inside the other.

Definition- A common tangent is one line tangent to two circles.