An arc is part of a circle's circumference.

In a circle, the degree measure of an arc is
equal to the measure of the central angle that intercepts the
arc. 



In a circle, the length of an arc is
a portion of the circumference.
Remembering that the arc measure is the measure
of the central angle, a definition can be formed as:


Example:
Understanding how an arc is measured
makes the next theorems common sense.

In the same circle, or congruent
circles, congruent central angles have congruent arcs. 


In the same circle, or congruent
circles, congruent arcs have congruent central angles. 

Remember: In the same circle, or congruent circles,
congruent arcs have congruent chords. Knowing this theorem makes
the next theorems seem straight forward.

In the same circle, or congruent
circles, congruent central angles have congruent chords. 


In the same circle, or congruent
circles, congruent chords have congruent central angles. 

