The longest diameter of an ellipse; the line from one side of an ellipse to the other that passes through the foci. Also, the length of that line.

the length of an ellipse measured through the foci from one side of the ellipse to the other

The line segment containing the foci whose endpoints are on the ellipse. Alternatively, the longest diameter of the ellipse.

The longer axis of an ellipse.

The maximum diameter of an ellipse.

() The axis passing through the focii of an ellipse. The largest diameter distance between opposite points of an ellipse.

(n) The long axis of an ellipse. In a pictorial projection, this axis represents the true diameter of the circle being projected.