The Free On-line Dictionary of Computing (17 December 2009):

Compatible Timesharing System
CTSS

    (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)


The Jargon File (version 4.4.7, 29 Dec 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


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

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


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

CTSS
       Cray TimeSharing System (OS, Cray, LLNL)