\begin{definition}[Definition:Ellipse/Focus-Directrix]
400px
Let $D$ be a straight line.
Let $F$ be a point.
Let $e \in \R: 0 < e < 1$.
Let $K$ be the locus of points $b$ such that the distance $p$ from $P$ to $D$ and the distance $q$ from $P$ to $F$ are related by the condition:
:$e p = q$
Then $K$ is an '''ellipse'''.
\end{definition}