\begin{definition}[Definition:Pi/Definition 1]
{{:Definition:Pi}}
Take a circle in a plane whose circumference is $C$ and whose radius is $r$.
Then $\pi$ can be defined as $\pi = \dfrac C {2r}$.
{{refactor|Include the below in a discussion on tau: $\tau {{=}} 2 \pi$}}
(It can be argued that $\pi = \dfrac C d$, where $d$ is the circle's diameter, is a simpler and more straightforward definition. However, the radius is, in general, far more immediately "useful" than the diameter, hence the above more usual definition in terms of circumference and radius.)
\end{definition}