directrix

The directrix is a fundamental concept in the definition of conic sections, including parabolas, ellipses, and hyperbolas. In mathematics, a directrix is a fixed line used in defining these curves.12345

For a parabola, the directrix is a fixed line, and the parabola is the set of all points that are equidistant from this directrix and a fixed point called the focus. This definition is crucial for understanding the shape and properties of a parabola.3

In the broader context of conic sections, the directrix, along with a focus, defines a curve based on a constant ratio of distances. This ratio, known as eccentricity, determines whether the conic section is an ellipse, parabola, or hyperbola.2