Definition:Parabola/Directrix

Definition

 * ParabolaFocusDirectrix.png

Let $K$ be an parabola specified in terms of:
 * a given straight line $D$
 * a given point $F$

where $K$ is the locus of points $P$ such that the distance $p$ from $P$ to $D$ equals the distance $q$ from $P$ to $F$:
 * $p = q$

The line $D$ is known as the directrix of the parabola.