Multics was a multitasking, multiuser operating system developed at MIT; its initial installation was made available primary to MIT users, including students. For many students it was their first interactive computer experience, allowing logging on through terminals instead of submitting a batch of cards. There were some other Multics installations, though it was never widely distributed. The Multics project began in 1964. It ran on GE (later Honeywell) hardware.
One of its most distinctive features was the definition of multiple levels of permitted access, called "protection rings," allowing fine-grained security control. Multics was an extremely ambitious design for its time and gave rise to ideas used in subsequent operating systems.
Unix is sometimes believed to have been named as a pun on Multics.