A parabola is an open curve in the plane, defined as the set of all points whose distances from a fixed point, called the focus, and a fixed line, called the directrix, are equal.
\[(x-h)^2 = 4p(y-k)\]
where the point \((h,k)\) is the vertex of the parabola.
A parabola has important reflection properties. See the article on conics for an exposition.