Convex lens: principal axis
The principal axis of a convex lense is the "main" axis.
It is an imaginary line that is perpendicular (at 90 degrees) to the verticle axis and passes through the centre of curvatures (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 the possible axes. Compare this to the principal singer of an opera, who is the most important singer.
Here we have the principal axis in red.