相切的英文：
tangency

A straight line tangent to a curve
与曲线相切的直线

tangent circles
相切圆,两个相切的圆

Lines which are everywhere tangential to the direction of the flow of electric current are called streamlines
处处均与电流方向相切的曲线称为流线。

A contact, as between two curves or surfaces, at three or more common points.
密切两段弧线或两个平面在三个或更多公共点处的相切