UNIX is a multi-user, multi-task, operating system developed in 1969 by Ken Thompson and Dennis Ritchie in Bell laboratories, which at the time were owned by AT&T and its subsidiary Western Electric.
This system has undergone many changes; the main one being its writing in C language in the first half of the 1970s, allowing it to be easy to port to any type of processor. Most of the big manufacturers went on to offer their own version of UNIX: IBM's AIX, Hewlett-Packard's HP-UX, Sun's Solaris, etc.
It inspired the development of the Linux free open source system. The name UNIX is a nod to its predecessor, MULTICS (Multiplexed Information and Computing Service), developed at the end of the 1960s at the Massachusetts Institute of Technology.
It was the first system to use the layer operating system model which is still in use today. Originally, UNIX was written UNICS, for UNIplexed Information and Computing Service.