An SOS was an ancient Earth term for a type of distress call. Simply Morse code for "S·O·S", this signal was displayed with the sound of three dots followed by three dashes followed by three more dots (··· −−− ···). (ENT: "Vanishing Point")
This call dates back to the early 20th century, when broadcast signaling was still a new science. An international convention met to debate and select a "universal" distress call, and "SOS" was selected by most. It became a universal call for assistance in 1912, after the sinking of the RMS Titanic.