Multics (Multiplexed Information and Computing Service) is an influential early time-sharing operating system, based around the concept of a single-level memory. Virtually all modern operating systems are heavily influenced by Multics; often through Unix, either directly (Linux, macOS) or indirectly (Microsoft Windows).
The design and features of Multics greatly influenced the Unix operating system, which was originally written by two Multics programmers, Ken Thompson and Dennis Ritchie. Superficial influence of Multics on Unix is evident in many areas, including the naming of some commands. But the internal design philosophy was quite different, focusing on keeping the system small and simple, and so correcting some deficiencies of Multics because of its high resource demands on the limited computer hardware of the time.
The name Unix (originally Unics) is itself a pun on Multics. The U in Unix is rumored to stand for uniplexed as opposed to the multiplexed of Multics, further underscoring the designers' rejections of Multics' complexity in favor of a more straightforward and workable approach for smaller computers. (Garfinkel and Abelson cite an alternative origin: Peter Neumann at Bell Labs, watching a demonstration of the prototype, suggested the name/pun UNICS (pronounced "Eunuchs"), as a "castrated Multics", although Dennis Ritchie is claimed to have denied this.)
Ken Thompson, in a transcribed 2007 interview with Peter Seibel refers to Multics as "...overdesigned and overbuilt and over everything. It was close to unusable. They (i.e., Massachusetts Institute of Technology) still claim it’s a monstrous success, but it just clearly wasn't." He admits, however, that "the things that I liked enough (about Multics) to actually take were the hierarchical file system and the shell—a separate process that you can replace with some other process."