Definition:Stationary Point

Definition
Let $f$ be a real function which is differentiable on the open interval $\left({a \,.\,.\, b}\right)$.

Let $\exists \xi \in \left({a \,.\,.\, b}\right): f^{\prime} \left({\xi}\right) = 0$, where $f^{\prime} \left({\xi}\right)$ is the derivative of $f$ at $\xi$.

Then $\xi$ is known as a stationary point of $f$.