Geom. the path of a point that moves so that the sum of its distances from two fixed points, the foci, is constant; closed curve formed by the section of a cone cut by a plane less steeply inclined than the side of the cone
Origin:
ModL ellipsis < Gr elleipsis, a defect, ellipse < elleipein, to fall short < en-, in + leipein, to leave (see loan): so named from falling short of a perfect circle