Definition:Conic Section/Directrix

Definition
Let $K$ be a conic section specified in terms of:
 * a given straight line $D$
 * a given point $F$
 * a given constant $\epsilon$

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 line $D$ is known as the directrix of the conic section.