UNIX (all caps) is the trademark for the original Unix operating system developed by Bell Labs/AT&T. The trademark was then transferred to The Open Group in 1993. However, the term is more generally used as a more generic name for all Unix-like operating systems, including Linux.
The original name was UNICS, which was a play on the name of the mainframe operating system that preceded it, which was called MULTICS, and which the Unix developers believed was excessively complex. The pun was that UNICS, or UNiplexed Information and Computing Service, was thus an "emasculated" Multics. It also isn't clear who changed the name to UNIX. So, while the original developers always capitalized Unix as if it were an acronym, it is not a true acronym.
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