The curve traced by a point on the circumference of a circle that rolls on a straight line.
圆滚线, 摆线圆周上任一点在一条直线上滚动时所形成的曲线
The distance around something; the circumference.
周围, 四周围绕某物的距离; 周围
The circumference of the earth is almost 25,000 miles.
地球的圆周长约为两万五千英里。