\begin{definition}[Definition:Midpoint-Convex]
Let $f$ be a real function defined on a real interval $I$.
$f$ is '''midpoint-convex''' {{iff}}:
:$\forall x, y \in I: f \left({\dfrac {x + y} 2}\right) \le \dfrac {f \left({x}\right) + f \left({y}\right)} 2$
\end{definition}