A plane curve such that the sums of the distances of each point in its periphery from two fixed points, the foci, are equal. It is a conic section formed by the intersection of a right circular cone by a plane that cuts the axis and the surface of the cone. Typical equation: (x2/a2) + (y2/b2) = 1. If a = b the ellipse is a circle. An ellipse is formed by a plane intersecting a cone at an angle to its base. All ellipses have two focal points or foci. The sum of the distances from every point on the ellipse to the two foci is constant. All ellipses have a centre and a major and minor axis.