principal axis
optic axis

Also parallel to the principal axis is one direction of perfect cleavage.

The directions x, y and z are in this case known as the principal axes of the medium.

These projects are to take place along five principal axes (or corridors) of development:

Indeed, even the principal axis of the gardens themselves was conceived to radiate from this fulcrum.

These directions are usually related to the principal axes of its crystal lattice.

The principal axis is crossed by one or more perpendicular perspectives and alleys.

The first constraint will be to put each torque in terms of the principal axes.

It is obtained by rotating an ellipse about one of its principal axes.

In each case, the x and y axes are the principal axes.

The principal axes are the lines spanned by the eigenvectors.