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By definition, a sphere is made by turning a semicircle around a straight line.

That straight line is called the axis of the sphere.

In the words of Euclid:

The axis of the sphere is the straight line which remains fixed about which the semicircle is turned.

(The Elements: Book $\text{XI}$: Definition $15$)

Linguistic Note

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

Compare basis.