A semicircle is half of a circle. A central angle which is subtended by a semicircle has a measure of 180°.

A circle is associated with a complete rotation, which is 360°.

A semi-circle is associated with half of a rotation which is 180°.

Minor arcs are associated with less than half of a rotation, so minor arcs are associated with angles less than 180°.

Major arcs are associated with more than half of a rotation, so major arcs are associated with angles greated than 180°.

Since the point of rotation is always the centre of the circle, CENTRAL ANGLES are used to describe the "measure" associated with an arc.