Definition:Right Circular Cone/Axis

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Let $K$ be a right circular cone.

Let point $A$ be the apex of $K$.

Let point $O$ be the center of the base of $K$.

Then the line $AO$ is the axis of $K$.

In the words of Euclid:

The axis of the cone is the straight line which remains fixed and about which the triangle is turned.

(The Elements: Book $\text{XI}$: Definition $19$)

Linguistic Note

The plural of axis is axes, which is pronounced ax-eez not ax-iz.

Compare basis.