The author presents a fairly simple physical interpretation of "m splitting" of the frequencies of nonradial oscillations in slowly rotating stars. This lifting of the degeneracy with respect to the azimuthal "quantum number" m by slow rotation is interpreted in terms of azimuthal running waves whose phase velocities are affected by the rotation. Hence, in a slowly rotating star the observed oscillation frequency depends on m. Another type of frequency splitting by rotation in stars whose pulsation and rotation axes do not coincide is also described briefly. Finally, the author presents a summary of astronomical bodies in which m splitting has been observed.