\begin{definition}[Definition:Ellipse/Eccentricity]
Let $K$ be an ellipse specified in terms of:
:a given straight line $D$
:a given point $F$
:a given constant $e$ such that $0 < e < 1$
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:
:$q = e p$
The constant $e$ is known as the '''eccentricity''' of the ellipse.
\end{definition}