Conic sections  Ellipse
An ellipse can be defined as the shape created when a plane intersects a cone at an angle to the cone's axis. It is one of the four conic sections. (the others are an circle, parabola and hyperbola).
In the above figure, there is a
plane*
that cuts through a
cone.
When the plane is parallel to the cone's base, the
intersection
of the cone and plane is a
circle.
But if the plane is tilted, the intersection becomes an
ellipse.
In the the figure above, as you drag the plane, you can create both a circle and an ellipse. The shape on the left shows the view that is perpendicular to the plane  as if you were looking straight down on the plane.
If you were to keep tilting the plane until it is parallel to the cone sides, the intersection would become a
parabola.
If you kept going until the plane was vertical, the intersection becomes a
hyperbola.
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