Chapter 13 - A Preview of Calculus: The Limit, Derivative, and Integral of a Function - Section 13.4 The Tangent Problem; The Derivative - 13.4 Assess Your Understanding - Page 916: 3

Tangent line.

Let $m_{tan}$ be the slope of the equation. Consider the tangent line that contains the point $(x_1,y_1)$. The slope the tangent line to the graph of $f(x)$ at $(x_1,y_1)$ can be written as $m_{tan} =\lim\limits_{x \to x_1} \dfrac{f(x)-f(x_1)}{x-x_1}$ Thus, the answer is "tangent line".