The *semi-major axis* is the length of the longest radius of an ellipse, that is the greatest distance between the centre of the ellipse and the perimeter of the ellipse.

For the ellipse with equation \[\frac{x^2}{a^2}+\frac{y^2}{b^2}=1\] with \(b\le a\), the semi-major axis is \(a\).

See also semi-minor axis.