\begin{definition}[Definition:Secant Line]
Let $f: \R \to \R$ be a real function.
Let the graph of $f$ be depicted on a Cartesian plane.
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A '''secant''' to $f$ is a straight line which intersects the graph of $f$ in (at least) two points.
In the above diagram, the '''secant''' is the line $AB$ in $\color {blue} {\text {blue} }$.
\end{definition}