\begin{definition}[Definition:Curvature/Whewell Form]
Let $C$ be a curve defined by a real function which is twice differentiable.
The '''curvature''' $\kappa$ of $C$ at a point $P$ can be expressed in the form of a Whewell equation as:
:$\kappa = \dfrac {\d \psi} {\d s}$
where:
:$\psi$ is the turning angle of $C$
:$s$ is the arc length of $C$.
\end{definition}