A powerful multi-tasking, multi-user computer operating system (OS). UNIX was developed at AT&T Bell Telephone Laboratories during the years 1969 to 1973 by Ken Thompson and Dennis Ritchie for minicomputer application. As UNIX is written in the highly portable C programming language, it is used on a wide variety of computers, from mainframes to PDAs. A number of UNIX variations have been developed, some of which are available as freeware, and are known as Linux. See also Bell Labs, freeware, Linux, mainframe computer, minicomputer, OS, PDA, and portability.