\begin{definition}[Definition:Point of Inflection/Definition 2]
Let $f$ be a real function which is differentiable on an interval $\Bbb I \subseteq \R$.
Let $\xi \in \Bbb I$.
$f$ has a '''point of inflection at $\xi$''' {{iff}} the derivative $f'$ of $f$ has either a local maximum or a local minimum at $\xi$.
\end{definition}