 Focal line

For a noncircular ellipse, the focal line is the straight line through its two distinct foci of a conic. It is the longer of the two principal axes of an ellipse. In principal-axis coordinates $x^2/a^2+y^2/b^2=1$, with $a>b>0$, the foci of a conic are $(\pm \sqrt{a^2-b^2},0)$, so the focal line is $y=0$. A circle has coincident foci of a conic at its center and therefore no distinguished focal line.