Теория:

Radius:
The radius is the distance from the center to any point on the circle.
radius.svg
  
Diameter:
The distance across the circle. The length of any chord passing through the center. It is twice the radius.
Diameter.svg
  
Chord:
The segment that joins the two points of the circle is called a chord.
Chord.svg
  
Tangent:
A line passing a circle and touching it at just one point.
all.svg