Major axis

The major axis of an ellipse is its longest axis. It passes through the foci of the ellipse. The perpendicular diameter which crosses the ellipse halfway between the foci is the minor axis. The semi-major axis is half the major axis and extends from the centre of the ellipse to one of its outermost points, passing through a focus. The axes are equivalent in an ellipse to the diameter of a circle, while the semi-axes are equivalent to radii.