On 10 June 1909, the 'SOS' signal was used in an emergency for the first time, when the Cunard liner SS Slavonia was wrecked off the Azores. Two steamers received her signals and went to the rescue.
An SOS is a Morse Code distress call which has been internationally recognized since 1906. It was mostly used by ships who wanted to be able to quickly and accurately signal each other to ask for assistance.
The Morse code international distress call (· · · — — — · · ·) was signaled via radio or using lights. On a radio which allows voice communications, people often used SOS or other terms like “mayday” to indicate that they are in trouble on ships, aircraft, and other vessels.
SOS has often been said to stand for such things as "Save Our Souls", "Save Our Ship", and "Stop Other Signals". However, these phrases are unofficial and came later, and are used as mnemonic devices to help remember the correct telegraphic pattern.