Focus
A focus is a point used to construct a conic section. (The plural is foci .) The focus points are used differently to determine each conic.
A circle is determined by one focus. A circle is the set of all points in a plane at a given distance from the focus (center).
A parabola is determined by a focus and a directrix (a line). A parabola is the set of points in a plane such that the distance from the focus equals the distance to the directrix.
An ellipse is determined by two foci. An ellipse is the set of points in a plane such that the sum of the distance from a point to each focus is constant.
A hyperbola is determined by two foci. A hyperbola is the set of points in a plane such that the difference between the distances from a point to each focus is constant.
