Definition:Cone (Geometry)

Definition
A cone is a three-dimensional geometric figure which consists of the set of all straight lines joining the perimeter of a plane figure $ABC$ to a point $P$ not in the plane of the figure:


 * Cone.png

Base
The plane figure $ABC$ is called the base of the cone.

Apex
The point $P$ is known as the apex of the cone.

Double Napped Cone
A double napped cone is one where the lines joining the apex to the perimeter of the base extend indefinitely in either dimension:


 * DoubleNappedCone.png

Comment
Hence the colloquial phrase "base over apex" as the description of a particularly flamboyant physical tumble.