A parabola is
the shape of the graph y = x^{2} , and of any quadratic function. It
is the approximate path followed by any projectile in flights such
as a shell, a ball or a stone. It has the property that when used
as a reflector it concentrates a parallel beam of light or radio
waves to a single point, the focus.

This fact is used in the design
of telescopes and television aerials for sending and receiving messages
from satellites. The shape taken up by the supporting cables of
a suspension bridge is parabolic, and some modern motorway bridges
have parabolic arches.

A parabola may
also be expressed as a conic section with an eccentricity of 1.
As a consequence of this, all parabolas are similar.

A parabola
can also be obtained as the limit of a sequence of ellipses where
one focus is kept fixed as the other is allowed to move arbitrarily
far away in one direction. In this sense, a parabola may be considered
an ellipse that has one focus at infinity. The parabola is an inverse
transform of a cardioid.