ctss

from Jargon File (4.4.4, 14 Aug 2003)

CTSS
 /C.T.S.S/, n.

   Compatible Time-Sharing System. An early (1963) experiment in the
   design of interactive timesharing operating systems, ancestral to
   {Multics}, {Unix}, and {ITS}. The name {ITS} (Incompatible
   Time-sharing System) was a hack on CTSS, meant both as a joke and to
   express some basic differences in philosophy about the way I/O
   services should be presented to user programs. See {timesharing}

from The Free On-line Dictionary of Computing (8 July 2008)

Compatible Timesharing System
CTSS

   <operating system> (CTSS) One of the earliest (1963)
   experiments in the design of interactive {time-sharing}
   {operating systems}.  CTSS was ancestral to {Multics}, {Unix},
   and {ITS}.  It was developed at the {MIT} Computation Center
   by a team led by Fernando J. Corbato.  CTSS ran on a modified
   {IBM 7094} with a second 32K-word bank of memory, using two
   {2301 drums} for swapping.  {Remote access} was provided to up
   to 30 users via an {IBM 7750} {communications controller}
   connected to {dial-up} {modems}.

   The name {ITS} (Incompatible {time-sharing} System) was a hack
   on CTSS, meant both as a joke and to express some basic
   differences in philosophy about the way I/O services should be
   presented to user programs.

   (1997-01-29)

from V.E.R.A. -- Virtual Entity of Relevant Acronyms (June 2006)

CTSS
       Compatible Time Sharing System (Unix, predecessor, OS, MIT)

from V.E.R.A. -- Virtual Entity of Relevant Acronyms (June 2006)

CTSS
       Cray TimeSharing System (OS, Cray, LLNL)