Definition:Stationary Point

Definition
Let $f$ be a real function which is differentiable on the open interval $\openint a b$.

Let $\exists \xi \in \openint a b: \map {f'} \xi = 0$, where $\map {f'} \xi$ is the derivative of $f$ at $\xi$.

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