a closed plane curve resulting from the intersection of a circular cone and a plane cutting completely through it

The sums of the distances from the foci to any point on an ellipse is constant.

source: WordNet 3.0

An oval or oblong figure, bounded by a regular curve, which corresponds to an oblique projection of a circle, or an oblique section of a cone through its opposite sides. The greatest diameter of the ellipse is the major axis, and the least diameter is the minor axis. See Conic section, under Conic, and cf. Focus.