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, becoming effective on July 1, 1908. SOS remained the maritime distress signal until 1999, when it was replaced by the Global Maritime Distress Safety System^[1].

From the beginning, the SOS distress signal has actually consisted of a continuous sequence of three-dits/three-dahs/three-dits, all run together without letter spacing. In International Morse Code, three dits form the letter S, and three dahs make the letter O, so "SOS" became an easy way to remember the correct order of the dits and dashes. In modern terminology, SOS is a Morse "procedural signal" or "prosign", and the formal way to write it is with a bar above the letters, i.e. SOS.

In popular usage, SOS became associated with phrases such as "Save Our Ship" and "Save Our Souls". These were a later development, most likely used to help remember the correct letters (something known as a backronym).