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