SoS

n. letters which represent the radiotelegraphic signal (... --- ...) used by ships in distress; any cry for help

SOS is the commonly used description for the International Morse code distress signal (· · · — — — · · ·). This distress signal was first adopted by the German government in radio regulations effective April 1, 1905, and became the worldwide standard when it was included in the second International Radiotelegraphic Convention, which was signed on November 3, 1906, and became effective on July 1, 1908.
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Noun
1. an internationally recognized distress signal in radio code
(hypernym) distress signal, distress call

S.O.S., distress signal, sign of danger or trouble, cry for help

S.O.S., letters which represent the radiotelegraphic signal (... --- ...) used by ships in distress; any cry for help