Concave lens – principal axis
The principal axis of a concave lens is the "main axis".
It is an imaginary line that is perpendicular (at 90°) to the vertical axis and passes through the centres of curvature (centre points of the spheres).
The principal opera singer is the main opera singer.
The word principal is used because it is the most important of all possible axes. Compare this to the principal singer of an opera. Here, she too is the most important singer.
Here we have the principal axis in red.