An arc is a portion of a circle. It could be a portion of some other curved shape, such as an ellipse, but it almost always refers to a circle. If the arc length is precisely half the circle, it is called a semicircular arc.
Arc symbolizes beginning and renewal in many cultures. Walking through an archway represents leaving the old and moving into a new phase of life. Aches are also symbolic of the extensiveness of the sky.
The semicircular arc represents the soul and mind, a half perfect, half-divine, and changing part of a human being. The figure is perceived to be broken and having visible and invisible portions. It represents something that is half-conscious and half unconscious.