UNIX, in computer science, a powerful multiuser, multitasking operating system. Considered a very powerful operating system, UNIX is written in the C language and can be installed on virtually any computer.

UNIX was originally developed by Ken Thompson and Dennis Ritchie at AT&T Bell Laboratories in 1969 for use on minicomputers. In the early 1970s, many universities, research institutions, and companies began to expand on and improve UNIX. These efforts resulted in two main versions: BSD UNIX, a version developed at the University of California at Berkeley, and System V, developed by AT&T and its collaborators.

Many companies developed and marketed their own versions of UNIX in subsequent years. Variations of UNIX include AIX, a version of UNIX adapted by IBM to run on RISC-based workstations; A/UX, a graphical version for the Apple Macintosh; XENIX OS, developed by Microsoft Corporation for 16-bit microprocessors; SunOS, adapted and distributed by Sun Microsystems, Inc.; Mach, a UNIX-compatible operating system for the NeXT computer; and Linux, developed by Finnish computer engineer Linus Torvalds with collaborators worldwide.