A specialization of both perimeter and portion of a line (qq.v.). Each instance of ellipse is a closed, one-dimensional curve that is the path of a point that moves so that the sum of its respective distances from two fixed points -- the ellipse's foci -- is constant.

An important specialization of ellipse is circle, each instance of which is an ellipse such that the distance between its foci is zero.

See also oval, each instance of which is a two-dimensional region whose boundary is an ellipse. And cf. ellipsoid.