Source : Webster's Revised Unabridged Dictionary (1913)

SOS \SOS\
   The letters signified by the signal ( . . . --- . . . )
   prescribed by the International Radiotelegraphic Convention
   of 1912 for use by ships in distress.

Source : WordNet®

SOS
     n : an internationally recognized distress signal in radio code

Source : Free On-Line Dictionary of Computing

SOS
     
        1. {Scheme Object System}.
     
        2. An infamously {losing} text editor.  Once, back in the
        1960s, when a text editor was needed for the {PDP-6}, a hacker
        crufted together a {quick-and-dirty} "stopgap editor" to be
        used until a better one was written.  Unfortunately, the old
        one was never really discarded when new ones (in particular,
        {TECO}) came along.  SOS is a descendant ("Son of Stopgap") of
        that editor, and many {PDP-10} users gained the dubious
        pleasure of its acquaintance.  Since then other programs
        similar in style to SOS have been written, notably the early
        font editor BILOS /bye'lohs/, the Brother-In-Law Of Stopgap
        (the alternate expansion "Bastard Issue, Loins of Stopgap" has
        been proposed).
     
        3. The {PDP-10} instruction to decrease a value.  Oppose
        {AOS}.
     
        [{Jargon File}]