\begin{definition}[Definition:Conic Section/Intersection with Cone/Circle/Transverse Section]
{{:Definition:Conic Section/Intersection with Cone}}
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Let $\phi = \dfrac \pi 2$, thereby making $D$ perpendicular to the axis of $C$.
The plane $D$ which is parallel to $B$, whose intersection with the cone is a circle, is known as a '''transverse section''' of the cone.
\end{definition}