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

Moby Thesaurus II by Grady Ward, 1.0:

44 Moby Thesaurus words for "SOS":
   Klaxon, Mayday, air-raid alarm, alarm, alarm bell, alarm clock,
   alarm signal, alarum, alert, all clear, beacon, blinking light,
   burglar alarm, buzzer, crostarie, fiery cross, fire alarm,
   fire bell, fire flag, five-minute gun, flashing light, fog bell,
   fog signal, foghorn, gale warning, hooter, horn, hue and cry,
   hurricane warning, lighthouse, note of alarm, occulting light,
   police whistle, signal of distress, siren, small-craft warning,
   still alarm, storm cone, storm flag, storm warning, tocsin,
   two-minute gun, upside-down flag, whistle



V.E.R.A. -- Virtual Entity of Relevant Acronyms (February 2016):

SOS
       Share Operating System (OS)


V.E.R.A. -- Virtual Entity of Relevant Acronyms (February 2016):

SOS
       Sophisticated Operating System (OS, Apple)


V.E.R.A. -- Virtual Entity of Relevant Acronyms (February 2016):

SOS
       Standards and Open Systems


V.E.R.A. -- Virtual Entity of Relevant Acronyms (February 2016):

SOS
       Support On Site


V.E.R.A. -- Virtual Entity of Relevant Acronyms (February 2016):

SOS
       Symbolic Operating System (OS)


The Free On-line Dictionary of Computing (30 December 2018):

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]