\begin{definition}[Definition:Ellipse/Major Axis/Semi-Major Axis]
:500px
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 $K$.
\end{definition}