Editing Focus (geometry) (section)

===Defining conics in terms of a focus and a directrix===
It is also possible to describe all [[conic section]]s in terms of a single focus and a single [[Conic section#Eccentricity, focus and directrix|directrix]], which is a given [[line (geometry)|line]] not containing the focus. A conic is defined as the locus of points for each of which the distance to the focus divided by the distance to the directrix is a fixed positive constant, called the [[eccentricity (mathematics)|eccentricity]] {{mvar|e}}. If {{math|0 < ''e'' < 1}} the conic is an ellipse, if {{math|1=''e'' = 1}} the conic is a parabola, and if {{math|''e'' > 1}} the conic is a hyperbola. If the distance to the focus is fixed and the directrix is a [[line at infinity]], so the eccentricity is zero, then the conic is a circle.