\begin{definition}[Definition:Point of Inflection/Definition 1]
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}} $\xi$ is a point on $f$ at which $f$ changes from being concave to convex, or vice versa.
\end{definition}