\begin{definition}[Definition:Conic Section/Focus-Directrix Property]
A '''conic section''' is a plane curve which can be specified in terms of:
:a given straight line $D$ known as the directrix
:a given point $F$ known as a focus
:a given constant $\epsilon$ known as the eccentricity.
Let $K$ be the locus of points $b$ such that the distance $p$ from $b$ to $D$ and the distance $q$ from $b$ to $F$ are related by the condition:
:$(1): \quad q = \epsilon \, p$
Then $K$ is a '''conic section'''.
Equation $(1)$ is known as the '''focus-directrix property''' of $K$.
\end{definition}