SOS is the commonly used description for the International Morse Code distress signal (· · · — — — · · ·). It was first adopted by the German government in radio regulations effective April 1, 1905, and became the worldwide standard under the second International Radiotelegraphic Convention, which was signed on November 3, 1906 and became effective on July 1, 1908. In 1999 the SOS radio distress signal was replaced by the Global Maritime Distress Safety System though SOS is still recognized as a visual distress signal.
In popular usage, SOS became associated with such phrases as “save our ship“, “save our souls” and “send out succour”. These may be regarded as mnemonics, but SOS is not an abbreviation or acronym. According to the Oxford English Dictionary (2nd ed.), the letters were chosen because they are easily transmitted in Morse code.