Definition:Ellipse/Major Axis/Semi-Major Axis

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Consider an ellipse $K$.

A semi-major axis of $K$ is either half of the major axis of $K$ from its midpoint to its endpoint.

In the above diagram, $O V_1$ and $O V_2$ are the semi-major axes of $K$.

Also known as

Some sources do not hyphenate: semimajor axis.

Also see

Linguistic Note

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

Compare basis.