For each focus of any conic section, there is a fixed line on the convex side, called the directrix, perpendicular to the axis of symmetry . For each point on the graph, its distance from the focus is directly proportional to its distance from the corresponding directrix. The proportionality constant is called the eccentricity , written .
The value of eccentricity determines the type of conic.
If , the conic is a circle .
If , the conic is an ellipse . (The closer is to zero, the more circular is the ellipse.)
If , the conic is a parabola .
If , the conic is a hyperbola .