“曲线的切线”是中学数学中一个基本的、重要的概念.教材中对切线定义的给出,是根据学生的认知情况逐步深化的,和历史上切线定义的演变基本上是一致的.本文对切线定义的演变过程作一些肤浅的研究.一、欧几里得切线定义关于切线的定义,最初在初中教材圆的知识中出现.圆的切线定义是:在平面内,和圆只有一个公共点的直线叫做圆的切线.在高中教材苏教版必修2解析几何中,讲直线与 The tangential curve is a basic and important concept in high school mathematics. The definition of tangents in textbooks is gradually deepened according to the students’ cognition and basically consistent with the historical evolution of tangential definitions .This paper does some superficial research on the evolution of tangential definition.First, the definition of tangential Euclidean tangent initially appeared in the knowledge of junior high school textbook circle tangent definition is: in the plane, and the circle Only a common point of the line is called the tangent of the circle. In the high school teaching materials Supian version of the required 2 analytic geometry, speaking straight and