\begin{definition}[Definition:Point-Circle]
A '''point-circle''' is the locus in the Cartesian plane of an equation of the form:
:$(1): \quad \paren {x - a}^2 + \paren {y - b}^2 = 0$
where $a$ and $b$ are real constants.
There is only one point in the Cartesian plane which satisfies $(1)$, and that is the point $\tuple {a, b}$.
It can be considered to be a circle whose radius is equal to zero.
\end{definition}