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.
Arc:
 a part of a circle's circumference.
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 arc 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).
Radii in a circle are congruent (of the same length). 