Image: density variation caused by irregular frame exposures or uneven printer speed.Audio: Rapid fluctuation in frequency caused by vibration in film or tape transport. 1 It is a distortion that occurs in sound reproduction as a result of undesired speed variations during recording or reproducing. Flutter occurring at frequencies below approximately 6 Hz is termed wow.
References
1 Case, D., 1985, Motion Picture Film Processing, Media Manuals, Focal Press.