A geometric element that generates a geometric figure, especially a straight line that generates a surface by moving in a specified fashion. 母点，母线形成一个几何图形的几何要素，尤指通过以规定方式运动而产生一个面的一条直线
A cold front is moving in from the north. 一股冷锋正从北方向这里移动。
The spacecraft is in orbit (ie moving in orbit) round the moon. 该航天器在绕月球轨道上飞行。
