A tangent line t to a circle C intersects the circle at a single point T. For comparison, secant lines ...

  

A tangent line t to a circle C intersects the circle at a single point T. For comparison, secant lines intersect a circle at two points, whereas another line may not intersect a circle at all. This property of tangent lines is preserved under many geometrical transformations, such as rotation, translations, inversions, and map projections, which is a great real world example, especially in the use of city maps.
 

There are many parts to a circle, what are some other parts of a circle and how do they relate to a circle?