# Definition:Hyperbola/Directrix

## Definition

Let $K$ be a hyperbola specified in terms of:

a given straight line $D$
a given point $F$
a given constant $\epsilon$ such that $\epsilon > 1$

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 hyperbola.