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A directrix is a fixed line used in the definition of conic sections. In the context of a parabola, a directrix is a line such that the distance from any point on the parabola to the focus is equal to the distance from that point to the directrix. This property forms the fundamental definition of a parabola. For ellipses and hyperbolas, the concept of a directrix is also employed, typically in conjunction with one or more foci. A point is on an ellipse or hyperbola if the ratio of its distance to a focus and its distance to the corresponding directrix is a constant value known as the eccentricity. The eccentricity determines the shape of the conic section. A parabola has an eccentricity of 1, an ellipse has an eccentricity less than 1, and a hyperbola has an eccentricity greater than 1. The position and orientation of the directrix are related to the position and orientation of the conic section itself. In Cartesian coordinates, the equation of a directrix is a linear equation representing a straight line. For example, for a parabola opening upwards with its vertex at the origin, the focus might be at (0, p) and the directrix would be the line y = -p.