Arc: a part of a circle's circumference.
Semicircle:
 an arc whose length is exactly half of the circumference (with a corresponding central angle equal to 180º).
Central angles: ∠AOC, ∠DOC, ∠AOJ, ∠DOJ, ∠AOD, ∠COJ

an angle whose vertex is at the center of the circle and whose rays (sides) are radii. 