\begin{definition}[Definition:Power of Point with respect to Circle]
Let $\CC$ be a circle embedded in the Cartesian plane with its center located at the origin.
Let $\CC$ have radius $r$.
Let $P = \tuple {x, y}$ be a point in the plane of $\CC$.
The '''power of $P$ with respect to $\CC$''' is the quantity:
:$x^2 + y^2 - r^2$
\end{definition}