\begin{definition}[Definition:Parabola/Focus-Directrix]
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Let $D$ be a straight line.
Let $F$ be a point.
Let $K$ be the locus of points $P$ such that the distance $p$ from $P$ to $D$ equals the distance $q$ from $P$ to $F$:
:$p = q$
Then $K$ is a '''parabola'''.
\end{definition}