Cone
In general, a cone is a pyramid with a circular cross section.
A right cone is a cone with its vertex above the center of its base.
However, when used without qualification, the term "cone" often means "right cone."
In discussions of conic sections, the word "cone" is taken to mean "double cone," i.e., two cones placed apex to apex. The double cone is a quadratic surface, and each single cone is called a "nappe." The hyperbola can then be defined as the intersection of a plane with both nappes of the cone.
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