| Focus of a Parabola The focus of a parabola is a fixed point on the interior of a parabola used in the formal definition of the curve. 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. Note: For a parabolic mirror, all rays of light emitting from the focus reflect off the parabola and travel parallel to each other (parallel to the axis of symmetry as well).  
| Example: | 
This is a graph of the parabola  with all its major features labeled: axis of symmetry, focus, vertex, and directrix. | See also Conic sections, foci of an ellipse, foci of a hyperbola, focal radius |
www.mathwords.com