\begin{definition}[Definition:Derivative/Higher Derivatives/Third Derivative]
Let $f$ be a real function which is twice differentiable on an open interval $I$.
Let $f''$ denote the second derivate.
Then the '''third derivative''' $f'''$ is defined as:
:$f''' := \dfrac {\d} {\d x} f'' = \map {\dfrac {\d} {\d x} } {\dfrac {\d^2} {\d x^2} f}$
Thus the '''third derivative''' is defined as the derivative of the second derivative.
If $f''$ is differentiable, then it is said that $f$ is '''triply differentiable''', or '''thrice differentiable'''.
\end{definition}