Definition:Conic Section/Eccentricity

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

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:
 * $q = e p$

The constant $e$ is known as the eccentricity of the conic section.