# Focus of a conic section

A conic section can be described as the locus of a point $P$ whose distance $PS$ from a fixed point $S$ is in some fixed ratio $e$ to its perpendicular distance $PN$ from a fixed line.

The point $S$ is called the focus of the conic, the fixed line is called the directrix of the conic, and the fixed ratio $e$ is called the eccentricity of the conic.