Definition:Ellipse/Major Axis/Semi-Major Axis

Definition

 * EllipseParts.png

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

 * Definition:Semi-Minor Axis of Ellipse