Definition:Conic Section/Intersection with Cone/Ellipse

Definition

 * ConicSections-Ellipse.png

Let $\theta < \phi < \dfrac \pi 2 - \theta$.

That is, the angle between $D$ and the axis of $C$ is between that for which $K$ is a circle and that which $K$ is a parabola.

Then $K$ is an ellipse.