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MOS is a standard filmmaking jargon abbreviation, used in production reports to indicate an associated film segment has no synchronous audio track. It stands for "motor only sync" or "motor only shot".
Omitting sound recording from a particular shot can save time and relieve the film crew of certain requirements, such as remaining silent during a take, and thus MOS takes are common on film shoots, most obviously when the subjects of the take are not speaking or otherwise generating useful sound.
Origins of the term 
There are many theories regarding the source of the abbreviation "MOS".
When sound recording reached the point where the sound was recorded on a synchronized but separate piece of media (such as 35mm film, audio tape, or other media) a method of keeping the recording media and camera film "in sync" was needed. The solution was to use a special form of motor which has multiple "windings" in it, and which can be connected to another identical motor in such a way that turning one motor a certain distance will turn the other motor exactly the same distance. The motors did not have to be close together, and, with appropriate circuitry, did not have to be of the same size or power. These motors were called selsyn (self synchronous) motors. A system was created where a single sound recording room could be connected to any of the stages on a studio lot (you can still see the connection points on some of the oldest stages.) The sound mixer (sound man) on stage connected the control panel to the recording room and the camera. There was a Selsyn motor on the camera and it was linked to a matching selsyn motor on the sound recording equipment at another point on the studio lot.
In order to use this system, the sound mixer used an intercom to the sound recordist to tell him to "roll", or start the system. Since this was a very mechanical system, it took some time to start and get up to proper speed. When proper speed and synchronization was reached, the recordist would use the intercom to announce, "Speed" and the sound mixer would relay that to the director and crew on the stage. The expression is still used, but now simply means, "Sound is recording".
It was the recordist who actually started and stopped the camera motor (the camera operator had a switch to ensure that the camera didn't roll at an inopportune time such as loading, replacing lenses, etc., and to stop it if something went amiss). The actual power source for the camera motor was in the sound booth.
When a shot was planned that did not require sound, the sound mixer would ask the recordist to "roll the motor only". The recordist would start the camera motor without starting the matching "sound" motor and electronics. The procedure acquired the name "motor only shot", thus MOS.
Alternative origins 
A popular (presumed to be mythical) origin theory is that MOS stands for broken English "mit out sound", that is, "without sound" as a 1920s German-émigré director might have said it.
According to this theory, a German director, recently transplanted to Hollywood (probably Ernst Lubitsch, but possibly Fritz Lang), was asked by a script supervisor how he would like to shoot the next scene of the day. The director responded "mit out sprechen!", and so this was noted as a joke on the production reports and the camera slates for the shot. In The Screenwriter's Bible, David Trottier credits the term to Austrian director Erich von Stroheim, who allegedly would tell his crew "Ve'll shoot dis mid out sound." Many have also referred to MOS as "motion on screen."
Documentary, news and reality shows have added to the meaning of MOS to mean "man on street" which are random interviews from the public, although in the UK the term "vox pop" is commonly used for this instead (from the Latin "vox populi"- "voice of the people").