A u-shaped curve with certain specific properties.
Formally, 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.
It is a common error to call any u-shaped curve a parabola. A
parabola must satisfy the conditions listed above, and a parabola
has a quadratic equation.
of a parabolic segment, focal radius