the area of the sector. Analyze the a circle. of Words to Know Write the letter of the definition next to the matching word as you work through the lesson. You may use the glossary to help you. central angle degree radian sector of a circle A. a standard unit of measure for angles; the measure of a central angle that subtends an arc that is equal in length to the radius of the circle B. the region of a circle bounded by two radii and their intercepted arc C. a unit of measure used in measuring angles and arcs; there are 360 degrees in a circle D. an angle whose vertex is at the center of a circle and whose sides are radii of that circle