Geometry/Circles/Radii, Chords and Diameters

A circle is a set of all points in a plane that are equidistant from a single point; that single point is called the centre of the circle and the distance between any point on circle and the centre is called radius of the circle.

http://en.wikipedia.org/upload/d/dd/Circle-2.png

Chord

a chord is an internal segment of a circle that has both of its endpoints on the circumference of the circle.
 * the diameter of a circle is the largest chord possible

Secant

a secant of a circle is any line that intersects a circle in two places.
 * a secant contains any chord of the circle

Tangent

a tangent to a circle is a line that intersects a circle in exactly one point, called the point of tangency.
 * at the point of tangency the tangent line and the radius of the circle are perpendicular