A line considered a limit to a curve in the sense that the perpendicular distance from a moving point on the curve to the line approaches zero as the point moves an infinite distance from the origin.

• 渐近线与曲线极限相关的一条直线，当曲线上某动点沿该曲线的一个分支移向无穷远时，动点到该直线的垂直距离趋于零