(追記) (追記ここまで)
Degenerate Conic Sections
Plane figures that can be obtained by the intersection of a double cone with a plane passing through the apex. These include a point, a line, and intersecting lines. Like other conic sections, all degenerate conic sections have equations of the form Ax2 + Bxy + Cy2 + Dx + Ey + F = 0.
See also