# Definition:Ellipse/Major Axis

## Definition

Consider an ellipse $K$ whose foci are $F_1$ and $F_2$.

The major axis of $K$ is the line segment passing through both $F_1$ and $F_2$ whose endpoints are where it intersects $K$.

In the above diagram, $V_1 V_2$ is the major axis of $K$.

### Semi-Major Axis

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