If I understand your description correctly, a line.
An ellipse is produced.
When a cone is sliced parallel to the base then the shape produced is a circle. If the cone is sliced at an angle so that the cut goes completely through the cone then an ellipse is produced. If the cut is made perpendicular to the cone's base then the shape produced is a parabola.
Conics, or conic sections, are the intersection of a plane with an infinite double cone. If that plane cuts both cones, it is a hyperbola. If it is parallel to the edge of the cone, you get a parabola. If neither is the case, it is an ellipse. The ellipse is also a circle if the plane is perpendicular to the altitude of the cone. Note that none of these are the case if the plane passes through the vertex of the cone.
A filter is a device to strain out substances you don't want from the stuff you do. A coffee filter strains out the grounds and lets the coffee pass through. An air filter strains out impurities in the air and lets the purer air through. A spam filter strains out the e-mails you don't want and lets the ones you do want through.
streamlining
A line is produced
An Ellipse
An ellipse is produced.
If a right circular cone is intersected by a plane so that the intersection goes through the cone's vertex as well as an edge of each nappe, the shape produced is a line. Not asked, but... If the angle of the plane is less than the angle of the cone, then the intersection is a point. If the angle of the plane is greater than the angle of the cone, then the intersection is two lines intersecting at the vertex. If the plane insersects at other than the vertex, then the intersection is a circle when the plane is perpendicular to the cone's axis, an ellipse when the plane's angle is less than the cone's angle, a parabola when the planes's angle equals the cone's angle, and two hyperbole's in the last case.
It will be a hyperbola.
hyperbola
hyperbola
The intersection forms a hyperbola.
CIRCULAR
Then the intersection is a hyperbola.
When it is produced it is processed through a tube, giving it a circular shape.
A traffic signal displaying a steady circular yellow light means that you should use caution when going through the intersection. You have the right away, but watch for other cars.