Definition:Derivative/Real Function/Derivative at Point/Definition 2

Definition
Let $I$ be an open real interval.

Let $f: I \to \R$ be a real function defined on $I$.

Let $\xi \in I$ be a point in $I$.

Let $f$ be differentiable at the point $\xi$. That is, suppose the limit $\displaystyle \lim_{h \mathop \to 0} \frac {\map f {\xi + h} - \map f \xi} h$ exists.

Then this limit is called the derivative of $f$ at the point $\xi$.

Also see

 * Equivalence of Definitions of Derivative