Vow of silence
A vow of silence or oath of silence was an oath made by an individual to not speak at all, or only to a limited number of people. The vow could be personal or religious in nature.

In 2369 after a Hunter found a Tosk in a holding cell aboard Deep Space 9 he asked him how he managed to get the help of these aliens without violating his oath of silence. ()

Hupyrian servants took partial vows of silence which forbade them from speaking to anyone other than to their Ferengi masters. ()

Tom Paris threatened to send The Doctor's character in the Fair Haven holoprogram,  Mulligan, to a nearby monastery with a vow of silence. ()

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de: Schweigegelübde
Vows