This is the best-known call for help at sea and in the air. It is sent in Morse code as three short sounds, three long sounds, and three short sounds, and anyone who picks it up knows the sender is in immediate danger. It behaves like a countable noun, so you send an SOS, hear an SOS, or pick up an SOS. The three letters do not stand for any phrase; they were chosen because the pattern is easy to tap, easy to hear and hard to mistake for anything else.

Here the word has left the sea and is used for any message that cannot wait: a radio announcement looking for a relative, a late-night text to a friend, a note pinned up at work. It keeps the feeling of the original signal, so it suggests real urgency rather than an ordinary request. Like the first sense it is countable: you send an SOS or send out an SOS. Choosing it instead of 'a request for help' tells the listener that the clock is running.