Radii in a circle are congruent.
Congruent circles have congruent radii.

A diameter is the longest possible chord.

Central angle:   

- an angle formed by two radii with the vertex at the center of the circle.

Secant:   
- a line passing through two points on the circle. A secant contains a chord in the interior of the circle.

Tangent:  
- a line touching (intersecting) a circle at one point without entering the interior of the circle.

Point of Tangency:    point A,  point J,  point D
- the point at which a tangent touches (intersects) a circle.
A radius drawn to the point of tangency is perpendicular to the tangent.

Arc:   
- a part of a circle's circumference.
The midpoint of an arc divides the arc into 2 congruent arcs.

Minor Arc:  
- any arc whose length is less than the length of a semicircle (with a corresponding central angle less than 180º).

Major Arc:  
- any arc whose length is greater than the length of a semicircle (with a corresponding central angle greater than 180º). Major arcs are usually named using three letters to distinguish them from minor arcs.

Semicircle:   
- an arc whose length is exactly half of the circumference (with a corresponding central angle equal to 180º).