Directrix
of a Parabola
A line perpendicular to
the axis
of symmetry used in the definition of a parabola.
A parabola is defined as follows: For a given point, called the
focus, and a given line
not through the focus, called the directrix, a parabola is the
locus of points such that the distance to the focus equals the
distance to the directrix.

See
also
Directrices of
an ellipse, directrices of a hyperbola
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