Definition:Differentiable Mapping/Real Function/Point

Definition
Let $f$ be a real function defined on an open interval $\openint a b$.

Let $\xi$ be a point in $\openint a b$.

Definition 2
These limits, if they exist, are called the derivative of $f$ at $\xi$.

Also see

 * Equivalence of Definitions of Differentiable Real Function at Point
 * Definition:Derivative of Real Function at Point