\begin{definition}[Definition:Ellipse/Focus]
Let $K$ be an ellipse specified in terms of:
:a given straight line $D$
:a given point $F$
:a given constant $\epsilon$ such that $0 < \epsilon < 1$
where $K$ is the locus of points $P$ such that the distance $p$ from $P$ to $D$ and the distance $q$ from $P$ to $F$ are related by the condition:
:$q = \epsilon \, p$
The point $F$ is known as the '''focus''' of the ellipse.
\end{definition}