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TOPS-10: /tops·ten/, n.

DEC's proprietary OS for the fabled PDP-10 machines, long a favorite of hackers but now long extinct. A fountain of hacker folklore; see Appendix A. See also ITS, TOPS-20, TWENEX, VMS, operating system. TOPS-10 was sometimes called BOTS-10 (from ‘bottoms-ten’) as a comment on the inappropriateness of describing it as the top of anything.