The semi-minor axis is the length of the shortest radius of an ellipse, that is the smallest 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-minor axis is \(b\).
See also semi-major axis.
