Definition:Parabola/Focus-Directrix

Definition

 * ParabolaFocusDirectrix.png

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.

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

Focus
The point $F$ is known as the focus of the parabola.

Also see

 * Equivalence of Definitions of Parabola