Definition:Stationary Point

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 $$f$$.