Footage recorded without sound but which is intended for use in a sound film; the appropriate noises are added later in production. If the footage involves an actor speaking and thus has to be dubbed, [[LipLock expect]] [[HongKongDub issues]] with [[MouthFlaps synchronizing]] the voice track.
The source of the initialism is debated; theories include "Motor Only Shot/Sync" (per [[http://en.wikipedia.org/wiki/MOS_%28filmmaking%29 TOW's article]]), "Missing Optical Sound", "Motion Omit Sound" and "[[GratuitousGerman Mitt Out Sound]]."
See also LoopingLines.
(Note: Some poster or another might've camelcased the abbreviation for ''Film/ManOfSteel'' into this. Was that what you were looking for?)