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.