## Precalculus (6th Edition) Blitzer

$focus$, $directrix$, $eccentricity$, $parabola$, $ellipse$, $hyperbola$.
Instead of defining conic sections separately, we have a universal definition that a conic section is the set of all points in the plane such that the ratio of the distance from a fixed point, called the $focus$, to the distance from a fixed line, called the $directrix$, equals to a constant $e$, called the $eccentricity$. If $e=1$, the conic is a $parabola$. If $e\lt1$, the conic is an $ellipse$. If $e\gt1$, the conic is a $hyperbola$.