Parabola

The American Heritage® Dictionary defines a parabola as:

"A plane curve formed by the intersection of a right circular cone and a plane parallel to an element of the cone or by the locus of points equidistant from a fixed line and a fixed point not on the line."

which doesn't mean very much to the average person. A practical definition is that a parabola is a surface that is curved such that incoming radiation (light, radio waves) striking its surface are reflected to a single point (or pretty close) called the focal point. It works in reverse as well; energy aimed from the focal point to the parabolic surface will be reflected back out in a well-aimed beam.

Satellite 'dishes' are probably the most familiar example of a parabolic reflector being used to focus energy (in this case, radio waves to the antenna element). Radio telescopes and solar concentrators are other examples.