An operating system invented in 1969 at AT&T Bell Labs. Copyrighted by AT&T, it was made available to researchers and students in 1973 and used to develop the Internet's communication software protocols. It's an interactive time-sharing system invented by Ken Thompson after Bell Labs left the Multics project, originally so he could play games on his scavenged PDP-7. Dennis Ritchie, the inventor of the programming language "C," is considered a coauthor of the system.
The turning point in Unix's history came when it was re-implemented almost entirely in C between 1972 and 1974, making it the first source-portable operating system. Unix subsequently underwent mutations and expansions at the hands of many different people, resulting in a uniquely flexible and developer-friendly environment. By 1991, Unix had become the most widely used multi-user, general-purpose operating system in the world. This kind of operating system is more popular for workstations, servers, and networks than for individual PCs.