Convex mirrors: principal axis
The principal axis of a convex mirror is the "main" axis. It is an imaginary line passing through the centre point of the sphere (centre of curvature) and through the centre of the mirror.
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 part.
Here we have the principal axis in red.