The Collaborative International Dictionary of English v.0.48:

Ellipse \El*lipse"\ ([e^]l*l[i^]ps"), n. [Gr. 'e`lleipsis,
   prop., a defect, the inclination of the ellipse to the base
   of the cone being in defect when compared with that of the
   side to the base: cf. F. ellipse. See Ellipsis.]
   1. (Geom.) 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.
      [1913 Webster]

   2. (Gram.) Omission. See Ellipsis.
      [1913 Webster]

   3. The elliptical orbit of a planet.
      [1913 Webster]

            The Sun flies forward to his brother Sun;
            The dark Earth follows wheeled in her ellipse.
                                                  --Tennyson.
      [1913 Webster]

WordNet (r) 3.0 (2006):

ellipse
    n 1: 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" [syn: ellipse, oval]

Moby Thesaurus II by Grady Ward, 1.0:

21 Moby Thesaurus words for "ellipse":
   arc, bow, catacaustic, catenary, caustic, circle, conchoid, crook,
   curl, curve, diacaustic, festoon, hook, hyperbola, lituus, oval,
   ovoid, ovule, parabola, sinus, tracery