BSD (Berkeley Software Distribution) a version of the UNIX
The Berkeley Software Distribution (BSD), also known as Berkeley Unix or BSD Unix, is a discontinued Unix operating system developed and distributed by the Computer Systems Research Group (CSRG) at the University of California, Berkeley beginning in 1978. It began as an improved derivative of AT&T's original Unix that was developed at Bell Labs, based on the source code but over time diverging into its own code. BSD would become a pioneer in the advancement of Unix and computing.
BSD's development was begun initially by Bill Joy, who added virtual memory capability to Unix running on a VAX-11 computer. In the 1980s, BSD was widely adopted by workstation vendors in the form of proprietary Unix distributions such as DEC Ultrix and Sun Microsystems SunOS due to its permissive licensing and familiarity to many technology company founders and engineers. It also became the most popular Unix at universities, where it was used for the study of operating systems. BSD was sponsored by DARPA until 1988, which led to the implementation of ARPANET and later the TCP/IP stack to Unix by BSD, which were released in BSD NET/1 in 1988. The codebase had been rewritten so much that as little as 5% of BSD contained original AT&T code, and therefore NET/1 was released without an AT&T source license.
The university ended its Unix research in 1992 following reduced funding as well as the Unix lawsuit. Since the original BSD has become obsolete, the term "BSD" is now commonly used for its open-source descendants, including FreeBSD, OpenBSD, NetBSD, and DragonFly BSD. BSD code have also served as the basis for Darwin and TrueOS; these, in turn, have been used by proprietary operating systems, including Apple's macOS and iOS, and Microsoft Windows which used (at least) part of its TCP/IP code. Code from BSD's open descendants have themselves also been used to create modern operating systems, for example the system software for the PlayStation 5 and others.