MOS film term
You may have heard a sound person say the MOS film term or seen it on the film slate or clapper as it's otherwise known. This relates to the shot being taken having no guide track or sync track for sound.
But where did the MOS film term come from? The MOS film term has been mainly attributed to early German directors in Hollywood coining the term. This is thought to be because there English wasn't perfect due to a W sounding like a V in German. So instead of saying Without Sound, being WS, the German directors would say Mit Out Sound, making MOS.
This is backed up only by the logic of not having a name for the actual process of no sound but what the result is. Some technicians would then use any of the following to describe the relevant technical process that was in operation:
Minus optical signal
Minus optical sound
Minus optical stripe
Muted on screen
Mute on sound
Mic off stage
Music on side
Motor only shot
Motor only sync
Most of these terms above relate to actual film which sadly isn't around as much anymore.
Most commonly you will hear Mute On Set or if you want to be a bit less serious Mixer Outside Smoking. Whatever your choice for MOS you now know it means no separate sync sound is to be recorded. There is a little wiggle room with having a guide mic on the camera which you would then call a Guide Track or GT on the slate.