What is Proper rotation axis, principal axis?

A proper rotation axis $C_n$ is a line about which a rotation by $2\pi/n$ is a symmetry operation, also written $C_n$; its powers $C_n^k$ are rotations by $2\pi k/n$, and $C_n^n = E$. The axis of highest $n$ is the principal axis, taken as $z$.