The Collaborative International Dictionary of English v.0.48:

SOS \SOS\
   The letters signified by the signal ( . . . --- . . . )
   prescribed by the International Radiotelegraphic Convention
   of 1912 for use by ships in distress.
   [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

SOS
    n 1: an internationally recognized distress signal in radio code

The Free On-line Dictionary of Computing (19 January 2023):

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]