what is an ellipse?

An ellipse is a plane curve that results from the intersection of a cone by a plane in a way that produces a closed curve.

Ellipses are closed curves and are the bounded case of the conic sections, the curves that result from the intersection of a circular cone and a plane that does not pass through its apex; the other two (open and unbounded) cases are parabolas and hyperbolas.

An ellipse is also the locus of all points of the plane whose distances to two fixed points add to the same constant.