In what units are angles measured? What is a circular arc.

Radian. An arc is one of two subsets of a circle into which any two different points belonging to it split it. Any two points A and B of the circle divide it into two parts; each of these parts is called an arc. If A and B are the ends of the diameter (i.e., the central angle of AOB is unfolded), then they define two equal arcs, called semicircles.

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