Answer
$focus$, $directrix$. $\frac{D(P-F)}{D(P-l)}$, $conic$ $section$.
$parabola$; $ellipse$; $hyperbola$. $eccentricity$.
Work Step by Step
We can generally describe a conic using a fixed point F called the $focus$ and a fixed line $l$ called the $directrix$. Points with a ratio of the distance $D$ from P to F and from P to line $l$ satisfying:
$\frac{D(P-F)}{D(P-l)}=e$ is a $conic$ $section$.
If $e=1$, the conic is a $parabola$; if $e\lt1$, the conic is an $ellipse$; and if $e\gt1$, the conic is a $hyperbola$. The number $e$ is call the $eccentricity$ of the conic.
